Drying wet fluoropolymer resin and exposing to oxygen source to reduce discoloration

ABSTRACT

Process for reducing thermally induced discoloration of fluoropolymer resin produced by polymerizing fluoromonomer in an aqueous dispersion medium to form aqueous fluoropolymer dispersion and isolating fluoropolymer from said aqueous medium by separating wet fluoropolymer resin from the aqueous medium and drying to produce fluoropolymer resin. The process comprises exposing the wet fluoropolymer resin to an oxygen source during drying.

FIELD OF THE INVENTION

This invention relates to a process for reducing thermally induceddiscoloration of fluoropolymer resin.

BACKGROUND OF THE INVENTION

A typical process for the aqueous dispersion polymerization offluorinated monomer to produce fluoropolymer includes feedingfluorinated monomer to a heated reactor containing an aqueous medium andadding a free-radical initiator to commence polymerization. Afluorosurfactant is typically employed to stabilize the fluoropolymerparticles formed. After several hours, the feeds are stopped, thereactor is vented and purged with nitrogen, and the raw dispersion inthe vessel is transferred to a cooling vessel.

The fluoropolymer formed can be isolated from the dispersion to obtainfluoropolymer resin. For example, polytetrafluoroethylene (PTFE) resinreferred to as PTFE fine powder is produced by isolating PTFE resin fromPTFE dispersion by coagulating the dispersion to separate PTFE from theaqueous medium and then drying. Dispersions of melt-processiblefluoropolymers such as copolymers of tetrafluoroethylene andhexafluoropropylene (FEP) and tetrafluoroethylene and perfluoro (alkylvinyl ethers) (PFA) useful as molding resins can be similarly coagulatedand the coagulated polymer is dried and then used directly inmelt-processing operations or melt-processed into a convenient form suchas chip or pellet for use in subsequent melt-processing operations.

Because of environmental concerns relating to fluorosurfactants, thereis interest in using hydrocarbon surfactants in the aqueouspolymerization medium in place of a portion of or all of thefluorosurfactant. However, when fluoropolymer dispersion is formed whichcontains hydrocarbon surfactant and is subsequently isolated to obtainfluoropolymer resin, the fluoropolymer resin is prone to thermallyinduced discoloration. By thermally induced discoloration is meant thatundesirable color forms or increases in the fluoropolymer resin uponheating. It is usually desirable for fluoropolymer resin to be clear orwhite in color and, in resin prone to thermally induced discoloration, agray or brown color, sometimes quite dark forms upon heating. Forexample, if PTFE fine power produced from dispersion containing thehydrocarbon surfactant sodium dodecyl sulfate (SDS) is converted intopaste-extruded shapes or films and subsequently sintered, an undesirablegray or brown color will typically arise. Color formation upon sinteringin PTFE produced from dispersion containing the hydrocarbon surfactantSDS has been described in Example VI of U.S. Pat. No. 3,391,099 toPunderson. Similarly, when melt-processible fluoropolymers such as FEPor PFA are produced from dispersions containing hydrocarbon surfactantsuch as SDS, undesirable color typically occurs when the fluoropolymeris first melt-processed, for example, when melt processed into aconvenient form for subsequent use such as chip or pellet.

SUMMARY OF THE INVENTION

The invention provides a process for reducing thermally induceddiscoloration of fluoropolymer resin produced by polymerizingfluoromonomer in an aqueous dispersion medium to form aqueousfluoropolymer dispersion and isolating fluoropolymer from said aqueousmedium by separating wet fluoropolymer resin from the aqueous medium anddrying to produce fluoropolymer resin. It has been discovered thatthermally induced discoloration of the fluoropolymer resin can bereduced by:

exposing the wet fluoropolymer resin to an oxygen source during drying.

Preferably, the process reduces the thermally induced discoloration byat least about 10% as measured by % change in L* on the CIELAB colorscale.

The process of the invention is useful for fluoropolymer resin whichexhibits thermally induced discoloration which ranges from mild tosevere. The process of the invention may be employed for fluoropolymerresin which exhibits thermally induced discoloration prior to treatmentwhich is significantly greater than equivalent fluoropolymer resin ofcommercial quality manufactured using ammonium perfluorooctanoatefluorosurfactant. The process of the invention is advantageouslyemployed when the fluoropolymer resin has an initial thermally induceddiscoloration value (L*_(i)) at least about 4 L units on the CIELABcolor scale below the L* value of equivalent fluoropolymer resin ofcommercial quality manufactured using ammonium perfluorooctanoatefluorosurfactant.

The invention is particularly useful for fluoropolymer resin obtainedfrom aqueous fluoropolymer dispersion made by polymerizing fluoromonomercontaining hydrocarbon surfactant which causes thermally induceddiscoloration, preferably aqueous fluoropolymer dispersion polymerizedin the presence of hydrocarbon surfactant.

DETAILED DESCRIPTION OF THE INVENTION

Fluoromonomer/Fluoropolymer

Fluoropolymer resins are produced by polymerizing fluoromonomer in anaqueous medium to form aqueous fluoropolymer dispersion. Thefluoropolymer is made from at least one fluorinated monomer(fluoromonomer), i.e., wherein at least one of the monomers containsfluorine, preferably an olefinic monomer with at least one fluorine or afluoroalkyl group attached to a doubly-bonded carbon. The fluorinatedmonomer and the fluoropolymer obtained therefrom each preferably containat least 35 wt % F, preferably at least 50 wt % F and the fluorinatedmonomer is preferably independently selected from the group consistingof tetrafluoroethylene (TFE), hexafluoropropylene (HFP),chlorotrifluoroethylene (CTFE), trifluoroethylene,hexafluoroisobutylene, perfluoroalkyl ethylene, fluorovinyl ethers,vinyl fluoride (VF), vinylidene fluoride (VF2),perfluoro-2,2-dimethyl-1,3-dioxole (PDD),perfluoro-2-methylene-4-methyl-1,3-dioxolane (PMD), perfluoro(allylvinyl ether) and perfluoro(butenyl vinyl ether) and mixtures thereof. Apreferred perfluoroalkyl ethylene monomer is perfluorobutyl ethylene(PFBE). Preferred fluorovinyl ethers include perfluoro(alkyl vinylether) monomers (PAVE) such as perfluoro(propyl vinyl ether) (PPVE),perfluoro(ethyl vinyl ether) (PEVE), and perfluoro(methyl vinyl ether)(PMVE). Non-fluorinated olefinic comonomers such as ethylene andpropylene can be copolymerized with fluorinated monomers.

Fluorovinyl ethers also include those useful for introducingfunctionality into fluoropolymers. These includeCF₂═CF—(O—CF₂CFR_(f))_(a)—O—CF₂CFR′_(f)SO₂F, wherein R_(f) and R′_(f)are independently selected from F, Cl or a perfluorinated alkyl grouphaving 1 to 10 carbon atoms, a=0, 1 or 2. Polymers of this type aredisclosed in U.S. Pat. No. 3,282,875 (CF₂═CF—O—CF₂CF(CF₃)—O—CF₂CF₂SO₂F,perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride)), and in U.S.Pat. Nos. 4,358,545 and 4,940,525 (CF₂═CF—O—CF₂CF₂SO₂F). Another exampleis CF₂═CF—O—CF₂—CF(CF₃)—O—CF₂CF₂CO₂CH₃, methyl ester ofperfluoro(4,7-dioxa-5-methyl-8-nonenecarboxylic acid), disclosed in U.S.Pat. No. 4,552,631. Similar fluorovinyl ethers with functionality ofnitrile, cyanate, carbamate, and phosphonic acid are disclosed in U.S.Pat. Nos. 5,637,748; 6,300,445; and 6,177,196.

A preferred class of fluoropolymers useful for reducing thermallyinduced discoloration is perfluoropolymers in which the monovalentsubstituents on the carbon atoms forming the chain or backbone of thepolymer are all fluorine atoms, with the possible exception ofcomonomer, end groups, or pendant group structure. Preferably thecomonomer, end group, or pendant group structure will impart no morethan 2 wt % C—H moiety, more preferably no greater than 1 wt % C—Hmoiety, with respect to the total weight of the perfluoropolymer.Preferably, the hydrogen content, if any, of the perfluoropolymer is nogreater than 0.2 wt %, based on the total weight of theperfluoropolymer.

The process of the present invention is also useful in reducingthermally induced discoloration of low molecular weight PTFE, which iscommonly known as PTFE micropowder, so as to distinguish from the PTFEdescribed above. The molecular weight of PTFE micropowder is lowrelative to PTFE, i.e. the molecular weight (Mn) is generally in therange of 10⁴ to 10⁵. The result of this lower molecular weight of PTFEmicropowder is that it has fluidity in the molten state, in contrast toPTFE which is not melt flowable. PTFE micropowder has melt flowability,which can be characterized by a melt flow rate (MFR) of at least 0.01g/10 min, preferably at least 0.1 g/10 min and more preferably at least5 g/10 min, and still more preferably at least 10 g/10 min., as measuredin accordance with ASTM D 1238, at 372° C. using a 5 kg weight on themolten polymer.

The invention is especially useful for reducing thermally induceddiscoloration of melt-processible fluoropolymers that are alsomelt-fabricable. Melt-processible means that the fluoropolymer can beprocessed in the molten state, i.e., fabricated from the melt usingconventional processing equipment such as extruders and injectionmolding machines, into shaped articles such as films, fibers, and tubes.Melt-fabricable means that the resultant fabricated articles exhibitsufficient strength and toughness to be useful for their intendedpurpose. This sufficient strength may be characterized by thefluoropolymer by itself exhibiting an MIT Flex Life of at least 1000cycles, preferably at least 2000 cycles, measured as disclosed in U.S.Pat. No. 5,703,185. The strength of the fluoropolymer is indicated by itnot being brittle.

Examples of such melt-processible fluoropolymers include homopolymerssuch as polychlorotrifluoroethylene and polyvinylidene fluoride (PVDF)or copolymers of tetrafluoroethylene (TFE) and at least one fluorinatedcopolymerizable monomer (comonomer) present in the polymer usually insufficient amount to reduce the melting point of the copolymersubstantially below that of PTFE, e.g., to a melting temperature nogreater than 315° C.

A melt-processible TFE copolymer typically incorporates an amount ofcomonomer into the copolymer in order to provide a copolymer which has amelt flow rate (MFR) of 0.1 to 200 g/10 min as measured according toASTM D-1238 using a 5 kg weight on the molten polymer and the melttemperature which is standard for the specific copolymer. MFR willpreferably range from 1 to 100 g/10 min, most preferably about 1 toabout 50 g/10 min. Additional melt-processible fluoropolymers are thecopolymers of ethylene (E) or propylene (P) with TFE or CTFE, notablyETFE and ECTFE.

A preferred melt-processible copolymer for use in the practice of thepresent invention comprises at least 40-99 mol % tetrafluoroethyleneunits and 1-60 mol % of at least one other monomer. Additionalmelt-processible copolymers are those containing 60-99 mol % PTFE unitsand 1-40 mol % of at least one other monomer. Preferred comonomers withTFE to form perfluoropolymers are perfluoromonomers, preferablyperfluoroolefin having 3 to 8 carbon atoms, such as hexafluoropropylene(HFP), and/or perfluoro(alkyl vinyl ether) (PAVE) in which the linear orbranched alkyl group contains 1 to 5 carbon atoms. Preferred PAVEmonomers are those in which the alkyl group contains 1, 2, 3 or 4 carbonatoms, and the copolymer can be made using several PAVE monomers.Preferred TFE copolymers include FEP (TFE/HFP copolymer), PFA (TFE/PAVEcopolymer), TFE/HFP/PAVE wherein PAVE is PEVE and/or PPVE, MFA(TFE/PMVE/PAVE wherein the alkyl group of PAVE has at least two carbonatoms) and THV (TFE/HFP/VF₂).

All these melt-processible fluoropolymers can be characterized by MFR asrecited above for the melt-processible TFE copolymers, i.e. by theprocedure of ASTM 1238 using standard conditions for the particularpolymer, including a 5 kg weight on the molten polymer in theplastometer for the MFR determination of PFA and FEP

Further useful polymers are film forming polymers of polyvinylidenefluoride (PVDF) and copolymers of vinylidene fluoride as well aspolyvinyl fluoride (PVF) and copolymers of vinyl fluoride.

The invention is also useful when reducing thermally induceddiscoloration of fluorocarbon elastomers (fluoroelastomers). Theseelastomers typically have a glass transition temperature below 25° C.and exhibit little or no crystallinity at room temperature and little orno melting temperature. Fluoroelastomer made by the process of thisinvention typically are copolymers containing 25 to 75 wt %, based ontotal weight of the fluoroelastomer, of copolymerized units of a firstfluorinated monomer which may be vinylidene fluoride (VF₂) ortetrafluoroethylene (TFE). The remaining units in the fluoroelastomersare comprised of one or more additional copolymerized monomers,different from the first monomer, selected from the group consisting offluorinated monomers, hydrocarbon olefins and mixtures thereof.Fluoroelastomers may also, optionally, comprise units of one or morecure site monomers. When present, copolymerized cure site monomers aretypically at a level of 0.05 to 7 wt %, based on total weight offluorocarbon elastomer. Examples of suitable cure site monomers include:i) bromine-, iodine-, or chlorine-containing fluorinated olefins orfluorinated vinyl ethers; ii) nitrile group-containing fluorinatedolefins or fluorinated vinyl ethers; iii) perfluoro(2-phenoxypropylvinyl ether); and iv) non-conjugated dienes.

Preferred TFE based fluoroelastomer copolymers include TFE/PMVE,TFE/PMVE/E, TFE/P and TFE/P/VF₂. Preferred VF₂ based fluorocarbonelastomer copolymers include VF₂/HFP, VF₂/HFP/TFE, and VF₂/PMVE/TFE. Anyof these elastomer copolymers may further comprise units of cure sitemonomer.

Hydrocarbon Surfactants

In one embodiment of the present invention, the aqueous fluoropolymerdispersion medium used to form fluoropolymer resin contains hydrocarbonsurfactant which causes thermally induced discoloration in the resinwhen the fluoropolymer resin is isolated and heated. The hydrocarbonsurfactant is a compound that has hydrophobic and hydrophilic moieties,which enables it to disperse and stabilize hydrophobic fluoropolymerparticles in an aqueous medium. The hydrocarbon surfactant is preferablyan anionic surfactant. An anionic surfactant has a negatively chargedhydrophilic portion such as a carboxylate, sulfonate, or sulfate saltand a long chain hydrocarbon portion, such as alkyl as the hydrophobicportion. Hydrocarbon surfactants often serve to stabilize polymerparticles by coating the particles with the hydrophobic portion of thesurfactant oriented towards the particle and the hydrophilic portion ofthe surfactant in the water phase. The anionic surfactant adds to thisstabilization because it is charged and provides repulsion of theelectrical charges between polymer particles. Surfactants typicallyreduce surface tension of the aqueous medium containing the surfactantsignificantly.

One example anionic hydrocarbon surfactant is the highly branched C10tertiary carboxylic acid supplied as Versatic® 10 by ResolutionPerformance Products.

Another useful anionic hydrocarbon surfactant is the sodium linear alkylpolyether sulfonates supplied as the Avanel® S series by BASF. Theethylene oxide chain provides nonionic characteristics to the surfactantand the sulfonate groups provide certain anionic characteristics.

Another group of hydrocarbon surfactants are those anionic surfactantsrepresented by the formula R-L-M wherein R is preferably a straightchain alkyl group containing from 6 to 17 carbon atoms, L is selectedfrom the group consisting of —ArSO₃ ⁻, —SO₃ ⁻, —SO₄ ⁻, —PO₃ ⁻, —PO₄ ⁻and —COO⁻, and M is a univalent cation, preferably H⁺, Na⁺, K⁺ and NH₄⁺. —ArSO₃ ⁻ is aryl sulfonate. Preferred of these surfactants are thoserepresented by the formula CH₃—(CH₂)_(n)-L-M, wherein n is an integer of6 to 17 and L is selected from —SO₄M, —PO₃M, —PO₄M, or —COOM and L and Mhave the same meaning as above. Especially preferred are R-L-Msurfactants wherein the R group is an alkyl group having 12 to 16 carbonatoms and wherein L is sulfate, and mixtures thereof. Especiallypreferred of the R-L-M surfactants is sodium dodecyl sulfate (SDS). Forcommercial use, SDS (sometimes referred to as sodium lauryl sulfate orSLS), is typically obtained from coconut oil or palm kernel oilfeedstocks, and contains predominately sodium dodecyl sulfate but maycontain minor quantities of other R-L-M surfactants with differing Rgroups. “SDS” as used in this application means sodium dodecyl sulfateor surfactant mixtures which are predominantly sodium docecyl sulphatecontaining minor quantities of other R-L-M surfactants with differing Rgroups.

Another example of anionic hydrocarbon surfactant useful in the presentinvention is the sulfosuccinate surfactant Lankropol® K8300 availablefrom Akzo Nobel Surface Chemistry LLC. The surfactant is reported to bethe following:

Butanedioic acid, sulfo-,4-(1-methyl-2-((1-oxo-9-octadecenyl)amino)ethyl)ester, disodium salt;CAS No.: 67815-88-7

Additional sulfosuccinate hydrocarbon surfactants useful in the presentinvention are diisodecyl sulfosuccinate, Na salt, available asEmulsogen® SB10 from Clariant, and diisotridecyl sulfosuccinate, Nasalt, available as Polirol® TR/LNA from Cesapinia Chemicals.

Another preferred class of hydrocarbon surfactants is nonionicsurfactants. A nonionic surfactant does not contain a charged group buthas a hydrophobic portion that is typically a long chain hydrocarbon.The hydrophilic portion of the nonionic surfactant typically containswater soluble functionality such as a chain of ethylene ether derivedfrom polymerization with ethylene oxide. In the stabilization context,surfactants stabilize polymer particles by coating the particles withthe hydrophobic portion of the surfactant oriented towards the particleand the hydrophilic portion of the surfactant in the water phase.

Nonionic hydrocarbon surfactants include polyoxyethylene alkyl ethers,polyoxyethylene alkyl phenyl ethers, polyoxyethylene alkyl esters,sorbitan alkyl esters, polyoxyethylene sorbitan alkyl esters, glycerolesters, their derivatives and the like. More specifically examples ofpolyoxyethylene alkyl ethers are polyoxyethylene lauryl ether,polyoxyethylene cetyl ether, polyoxyethylene stearyl ether,polyoxyethylene oleyl ether, polyoxyethylene behenyl ether and the like;examples of polyoxyethylene alkyl phenyl ethers are polyoxyethylenenonyl phenyl ether, polyoxyethylene octyl phenyl ether and the like;examples of polyoxyethylene alkyl esters are polyethylene glycolmonolaurylate, polyethylene glycol monooleate, polyethylene glycolmonostearate and the like; examples of sorbitan alkyl esters arepolyoxyethylene sorbitan monolaurylate, polyoxyethylene sorbitanmonopalmitate, polyoxyethylene sorbitan monostearate, polyoxyethylenesorbitan monooleate and the like; examples of polyoxyethylene sorbitanalkyl esters are polyoxyethylene sorbitan monolaurylate, polyoxyethylenesorbitan monopalmitate, polyoxyethylene sorbitan monostearate and thelike; and examples of glycerol esters are glycerol monomyristate,glycerol monostearate, glycerol monooleate and the like. Also examplesof their derivatives are polyoxyethylene alkyl amine, polyoxyethylenealkyl phenyl-formaldehyde condensate, polyoxyethylene alkyl etherphosphate and the like. Particularly preferable are polyoxyethylenealkyl ethers and polyoxyethylene alkyl esters. Examples of such ethersand esters are those that have an HLB value of 10 to 18. Moreparticularly there are polyoxyethylene lauryl ether (EO: 5 to 20. EOstands for an ethylene oxide unit), polyethylene glycol monostearate(EO: 10 to 55) and polyethylene glycol monooleate (EO: 6 to 10).

Suitable nonionic hydrocarbon surfactants include octyl phenolethoxylates such as the Triton® X series supplied by Dow ChemicalCompany:

Preferred nonionic hydrocarbon surfactants are branched alcoholethoxylates such as the Tergitol® 15-S series supplied by Dow ChemicalCompany and branched secondary alcohol ethoxylates such as the Tergitol®TMN series also supplied by Dow Chemical Company.:

Ethyleneoxide/propylene oxide copolymers such as the Tergitol® L seriessurfactant supplied by Dow Chemical Company are also useful as nonionicsurfactants in this invention.

Yet another useful group of suitable nonionic hydrocarbon surfactantsare difunctional block copolymers supplied as Pluronic® R series fromBASF, such as:

Another group of suitable nonionic hydrocarbon surfactants are tridecylalcohol alkoxylates supplied as Iconol® TDA series from BASFCorporation.

In a preferred embodiment, all of the monovalent substituents on thecarbon atoms of the hydrocarbon surfactants are hydrogen. Thehydrocarbon is surfactant is preferably essentially free of halogensubstituents, such as fluorine or chlorine. Accordingly, the monovalentsubstituents, as elements from the Periodic Table, on the carbon atomsof the surfactant are at least 75%, preferably at least 85%, and morepreferably at least 95% hydrogen. Most preferably, 100% of themonovalent substituents as elements of the Periodic Table, on the carbonatoms are hydrogen. However, in one embodiment, a number of carbon atomscan contain halogen atoms in a minor amount.

Examples of hydrocarbon-containing surfactants useful in the presentinvention in which only a minor number of monovalent substituents oncarbon atoms are fluorine instead of hydrogen are the PolyFox®surfactants available from Omnova Solutions, Inc., described below

Polymerization Process

For the practice of the present invention, fluoropolymer resin isproduced by polymerizing fluoromonomer. Polymerization may be suitablycarried out in a pressurized polymerization reactor which producesaqueous fluoropolymer dispersion. A batch or continuous process may beused although batch processes are more common for commercial production.The reactor is preferably equipped with a stirrer for the aqueous mediumand a jacket surrounding the reactor so that the reaction temperaturemay be conveniently controlled by circulation of a controlledtemperature heat exchange medium. The aqueous medium is preferablydeionized and deaerated water. The temperature of the reactor and thusof the aqueous medium will preferably be from about 25 to about 120° C.

To carry out polymerization, the reactor is typically pressured up withfluoromonomer to increase the reactor internal pressure to operatingpressure which is generally in the range of about 30 to about 1000 psig(0.3 to 7.0 MPa). An aqueous solution of free-radical polymerizationinitiator can then be pumped into the reactor in sufficient amount tocause kicking off of the polymerization reaction, i.e. commencement ofthe polymerization reaction. The polymerization initiator employed ispreferably a water-soluble free-radical polymerization initiator. Forpolymerization of TFE to PTFE, preferred initiator is organic peracidsuch as disuccinic acid peroxide (DSP), which requires a large amount tocause kickoff, e.g. at least about 200 ppm, together with a highlyactive initiator, such as inorganic persulfate salt such as ammoniumpersulfate in a smaller amount. For TFE copolymers such as FEP and PFA,inorganic persulfate salt such as ammonium persulfate is generally used.The initiator added to cause kickoff can be supplemented by pumpingadditional initiator solution into the reactor as the polymerizationreaction proceeds.

For the production of modified PTFE and TFE copolymers, relativelyinactive fluoromonomer such as hexafluoropropylene (HFP) can already bepresent in the reactor prior to pressuring up with the more active TFEfluoromonomer. After kickoff, TFE is typically fed into the reactor tomaintain the internal pressure of the reactor at the operating pressure.Additional comonomer such as HFP or perfluoro (alkyl vinyl ether) can bepumped into the reactor if desired. The aqueous medium is typicallystirred to obtain a desired polymerization reaction rate and uniformincorporation of comonomer, if present. Chain transfer agents can beintroduced into the reactor when molecular weight control is desired.

In one embodiment of the present invention, the aqueous fluoropolymerdispersion is polymerized in the presence of hydrocarbon surfactant.Hydrocarbon surfactant is preferably present in the fluoropolymerdispersion because the aqueous fluoropolymer dispersion is polymerizedin the presence of hydrocarbon surfactant, i.e., hydrocarbon surfactantis used as a stabilizing surfactant during polymerization. If desiredfluorosurfactant such a fluoroalkane carboxylic acid or salt orfluoroether carboxylic acid or salt may be employed as stabilizingsurfactant together with hydrocarbon surfactant and therefore may alsopresent in the aqueous fluoropolymer dispersion produced. Preferably forthe practice of the present invention, the fluoropolymer dispersion ispreferably free of halogen-containing surfactant such asfluorosurfactant, i.e., contains less than about 300 ppm, and morepreferably less than about 100 ppm, and most preferably less than 50ppm, or halogen-containing surfactant.

In a polymerization process employing hydrocarbon surfactant as thestabilizing surfactant, addition of the stabilizing surfactant ispreferably delayed until after the kickoff has occurred. The amount ofthe delay will depend on the surfactant being used and the fluoromonomerbeing polymerized. In addition, it is preferably for the hydrocarbonsurfactant to be fed into the reactor as the polymerization proceeds,i.e., metered. The amount of hydrocarbon surfactant present in theaqueous fluoropolymer dispersion produced is preferably 10 ppm to about50,000 ppm, more preferably about 50 ppm to about 10,000 ppm, mostpreferably about 100 ppm to about 5000 ppm, based on fluoropolymersolids.

If desired, the hydrocarbon surfactant can be passivated prior to,during or after addition to the polymerization reactor. Passivatingmeans to reduce the telogenic behavior of the hydrocarbon-containingsurfactant. Passivation may be carried out by reacting said thehydrocarbon-containing surfactant with an oxidizing agent, preferablyhydrogen peroxide or polymerization initiator. Preferably, thepassivating of the hydrocarbon-containing surfactant is carried out inthe presence of a passivation adjuvant, preferably metal in the form ofmetal ion, most preferably, ferrous ion or cuprous ion.

After completion of the polymerization when the desired amount ofdispersed fluoropolymer or solids content has been achieved (typicallyseveral hours in a batch process), the feeds are stopped, the reactor isvented, and the raw dispersion of fluoropolymer particles in the reactoris transferred to a cooling or holding vessel.

The solids content of the aqueous fluoropolymer dispersion aspolymerized produced can range from about 10% by weight to up to about65 wt % by weight but typically is about 20% by weight to 45% by weight.Particle size (Dv(50)) of the fluoropolymer particles in the aqueousfluoropolymer dispersion can range from 10 nm to 400 nm, preferablyDv(50) about 100 to about 400 nm.

Isolation of the fluoropolymer includes separation of wet fluoropolymerresin from the aqueous fluoropolymer dispersion. Separation of the wetfluoropolymer resin from the aqueous fluoropolymer dispersion can beaccomplished by a variety of techniques including but not limited togelation, coagulation, freezing and thawing, and solvent aidedpelletization (SAP). When separation of wet fluoropolymer resin iscarried out by coagulation, the as polymerized dispersion may first bediluted from its as polymerized concentration. Stirring is then suitablyemployed to impart sufficient shear to the dispersion to causecoagulation and thereby produce undispersed fluoropolymer. Salts such asammonium carbonate can be added to the dispersion to assist withcoagulation if desired. Filtering can be used to remove at least aportion of the aqueous medium from the wet fluoropolymer resin.Separation can be performed by solvent aided pelletization as describedin U.S. Pat. No. 4,675,380 which produces granulated particles offluoropolymer.

Isolating the fluoropolymer typically includes drying to remove aqueousmedium which is retained in the fluoropolymer resin. After wetfluoropolymer resin is separated from the dispersion, fluoropolymerresin in wet form can include significant quantities of the aqueousmedium, for example, up to 60% by weight. Drying removes essentially allof the aqueous medium to produce fluoropolymer resin in dry form. Thewet fluoropolymer resin may be rinsed if desired and may be pressed toreduce aqueous medium content to reduce the energy and time required fordrying.

For melt-processible fluoropolymers, wet fluoropolymer resin is driedand used directly in melt-processing operations or processed into aconvenient form such as chip or pellet for use in subsequentmelt-processing operations. Certain grades of PTFE dispersion are madefor the production of fine powder. For this use, the dispersion iscoagulated, the aqueous medium is removed and the PTFE is dried toproduce fine powder. For fine powder, conditions are suitably employedduring isolation which do not adversely affect the properties of thePTFE for end use processing. The shear in the dispersion during stirringis appropriately controlled and temperatures less than 200° C., wellbelow the sintering temperature of PTFE, are employed during drying.

Reduction of Thermally Induced Discoloration

To reduce thermally induced discoloration in accordance with the presentinvention, wet fluoropolymer resin is exposed to an oxygen source duringdrying. Preferably, the process of the invention reduces the thermallyinduced discoloration by at least about 10% as measured by % change inL* on the CIELAB color scale. As discussed in detail in the Test Methodswhich follow, the % change in L* of fluoropolymer resin samples isdetermined using the CIELAB color scale specified by InternationalCommission on Illumination (CIE). More preferably, the process reducesthe thermally induced discoloration by at least about 20% as measured by% change in L*, still more preferably at least about 30%, and mostpreferably at least about 50%.

The wet fluoropolymer resin for use in the practice of the invention ispreferably undispersed fluoropolymer as separated from the dispersion.

Any of various equipment known for use in drying fluoropolymer resin canbe used for the practice of the invention. In such equipment a heateddrying gas, typically air, is used as a heat transfer medium to heat thefluoropolymer resin and to convey away water vapor and chemicals removedfrom the fluoropolymer resin during drying. Preferably in accordancewith the present invention, the drying gas employed is the oxygen sourceor includes the oxygen source as discussed below.

The process of the invention can be carried out such that thefluoropolymer resin is dried under static conditions or dynamicconditions. “Static conditions” means that the fluoropolymer is notsubjected to agitation such as by stirring or shaking during dryingalthough drying in equipment such as tray drying in an oven result incirculation of the drying gas by convection. “Dynamic conditions” meansthat the process is carried while moving the fluoropolymer resin such asby stirring or shaking or actively passing a drying gas through thefluoropolymer resin which may additionally cause movement thefluoropolymer resin. Heat transfer and mass transfer can be facilitatedby the use of dynamic conditions, for example, flowing the drying gasthrough the polymer bed. Preferably, the process of the invention iscarried out under dynamic conditions. Preferred equipment and processconditions for drying under dynamic conditions is disclosed by Egres,Jr. et al. U.S. Pat. No. 5,391,709, in which the wet fluoropolymer resinis deposited as a shallow bed on fabric and dried by passing heated airthrough the bed, preferably from top to bottom.

As used in this application, “oxygen source” means any chemical sourceof available oxygen. “Available oxygen” means oxygen capable of reactingas an oxidizing agent. Preferably, the oxygen source is air, oxygen richgas, or ozone-containing gas. “Oxygen rich gas” means pure oxygen andgas mixtures containing greater than about 21% oxygen by volume,preferably oxygen enriched air. Preferably, oxygen rich gas contains atleast about 22% oxygen by volume. “Ozone containing gas” means pureozone and gas mixtures containing ozone, preferably ozone enriched air.Preferably, the content of ozone in the gas mixture is at least about 10ppm ozone by volume.

One preferred oxygen source for practice of the present invention isozone containing gas, preferably ozone enriched air. Ozone enriched airas the drying gas can be provided by employing an ozone generator whichfeeds ozone into the drying air as it is supplied to the dryingapparatus used. Another preferred oxygen source is oxygen rich gas,preferably oxygen enriched air. Oxygen enriched air as the drying gascan be provided by feeding oxygen into the drying air as it is suppliedto the drying apparatus used. Oxygen enriched air can also be providedby semipermeable polymeric membrane separation systems.

Temperatures of drying gas during drying can be in the range of about100° C. to about 300° C. Higher temperature drying gases shorten thedrying time and facilitate the reduction of thermally induceddiscoloration. However, temperatures of the drying gas should not causethe temperature of the fluoropolymer resin to reach or exceed itsmelting point which will cause the fluoropolymer to fuse. Formelt-processible fluoropolymers, preferred drying gas temperatures are160° C. to about 10° C. below the melting point of the fluoropolymer.The end use properties of PTFE resin can be adversely affected bytemperatures well below its melting point. Preferably, PTFE resin isdried using drying gas at a temperature of about 100° C. to about 200°C., more preferably, about 150° C. to about 180° C.

The time necessary to carry out the process of the invention will varywith factors including the thickness of the wet fluoropolymer resinbeing dried, the temperature employed, the oxygen source employed andthe rate of circulation of the drying gas. When ozone containing gas isused as the oxygen source, the reduction of thermally induceddiscoloration can be accomplished during normal drying times, preferablyin the range of about 15 minutes to 10 hours. If desired, the process ofthe invention can be continued after the fluoropolymer resin is dry forthe purposes of reducing thermally induced discoloration.

In a preferred form of the invention, the exposing of the wetfluoropolymer resin to an oxygen source during drying is carried out inthe presence of alkali metal salt. In this form of the invention, it ispreferred for the oxygen source to be ozone containing gas.

Any of variety of alkali metal salts can be used. The anion in thealkali metal salt preferably is non-reactive with components of thedispersion although hydroxide can be beneficial since higher pH valuescan promote reduction of discoloration. Preferred alkali metal salts aresodium salts, lithium salts and potassium salts with potassium saltsbeing most preferred. Examples of suitable alkali metal salts are KCl,K₂CO₃, K₂SO₄, NaOH, NaCl, LiOH and LiCl.

The presence of alkali metal salt during drying is preferably providedby adding alkali metal salt to the aqueous medium prior to separatingthe wet fluoropolymer resin from said aqueous medium. For example, thealkali metal salt can be added before coagulation or after coagulationbut before the aqueous medium is removed. Preferably, the alkali metalsalt is added prior to coagulation.

When added to the aqueous medium, the amount of alkali metal salt canvary depending upon the type of salt added, the severity of thediscoloration and other conditions employed in the process. Preferably,the amount of alkali metal salt added to the dispersion prior tocoagulation is about 5 ppm to about 50,000 ppm based on the weight ofdry fluoropolymer, more preferably about 50 ppm to about 25,000 ppm, andmost preferably about 150 ppm to about 10,000 ppm.

In a preferred form of the invention, the process further comprisespretreating the aqueous fluoropolymer dispersion, preferably by exposingthe aqueous fluoropolymer dispersion to an oxidizing agent. In thepractice of the present invention employing a pretreatment, thepretreatment may or may not result in reduction of thermally induceddiscoloration as measured by % change in L* if employed alone withoutsubsequently exposing the wet fluoropolymer resin to an oxygen sourceduring drying. Moreover, it is possible that the thermally induceddiscoloration of the fluoropolymer resin may be increased, i.e., thediscoloration worsens, by the pretreatment alone without subsequentlyexposing the wet fluoropolymer resin to an oxygen source during drying.However, the additive effect of the pretreatment in combination withexposing the wet fluoropolymer resin to an oxygen source during dryingin accordance with the invention preferably provides an improvement overthe reduction of thermally induced discoloration provided only byexposing the wet fluoropolymer resin to an oxygen source during drying.The reduction of thermally induced discoloration measured by % change inL* on the CIELAB color scale provided by pretreating in combination withexposing the wet fluoropolymer resin to an oxygen source during dryingis preferably at least about 10% greater than the % change in L* on theCIELAB color scale provided by only exposing the wet fluoropolymer resinto an oxygen source under the same conditions during drying, morepreferably 20% greater, still more preferably 30% greater, mostpreferably 50% greater.

Pretreatment of the aqueous fluoropolymer dispersion can be accomplishedby a variety of techniques. One preferred pretreatment comprisesexposing the aqueous fluoropolymer dispersion to ultraviolet light inthe presence of an oxygen source. For the practice of this pretreatment,the aqueous fluoropolymer dispersion is preferably first diluted withwater to a concentration less than the concentration of the aspolymerized aqueous fluoropolymer dispersion because, depending upon theequipment used, exposure of ultraviolet light can be more effective forreducing discoloration for dilute dispersions. Preferred concentrationsare about 2 weight percent to about 30 weight percent, more preferablyabout 2 weight percent to about 20 weight percent.

Ultraviolet light has a wavelength range or about 10 nm to about 400 nmand has been described to have bands including: UVA (315 nm to 400 nm),UVB (280 nm to 315 nm), and UVC (100 nm to 280 nm). Preferably, theultraviolet light employed has a wavelength in the UVC band.

Any of various types of ultraviolet lamps can be used as the source ofultraviolet light. For example, submersible UV clarifier/sterilizerunits sold for the purposes of controlling algae and bacterial growth inponds are commercially available and may be used for the practice ofthis pretreatment. These units include a low-pressure mercury-vapor UVClamp within a housing for the circulation of water. The lamp isprotected by a quartz tube so that water can be circulated within thehousing for exposure to ultraviolet light. Submersible UVclarifier/sterilizer units of this type are sold, for example, under thebrand name Pondmaster by Danner Manufacturing, Inc. of Islandia N.Y. Forcontinuous treatment processes, the dispersion can be circulated thoughunits of this type to expose the dispersion to ultraviolet light. Singlepass or multiple pass treatments can be employed.

Dispersion can also be processed in a batch operation in a containersuitable for exposure to ultraviolet light in the presence of an oxygensource. In this pretreatment, it is desirable for a suitably protectedultraviolet lamp to be immersed in the dispersion. For example, a vesselnormally used for coagulation of the aqueous fluoropolymer dispersion toproduce fluoropolymer resin can be used for carrying out the process ofthis pretreatment by immersing the ultraviolet lamp in the dispersionheld in this vessel. The dispersion can be circulated or stirred ifdesired to facilitate exposure to the ultraviolet light. When the oxygensource is a gas as discussed below, circulation may be achieved orenhanced by injecting the oxygen source into the dispersion. Ultravioletlamps with protective quartz tubes of the type employed in thesubmersible UV clarifier/sterilizer units can be employed for immersionin dispersion after being removed from their housing. Other ultravioletlamps such as medium-pressure mercury-vapor lamps can also be used withthe lamp suitably protected for immersion in the dispersion such as byenclosing the lamp in a quartz photowell. A borosilicate glass photowellcan also be used although it may decrease effectiveness by filteringultraviolet light in the UVC and UVB bands. Suitable medium-pressuremercury vapor lamps are sold by Hanovia of Fairfield, N.J.

As used for this pretreatment, “oxygen source” means any chemical sourceof available oxygen. “Available oxygen” means oxygen capable of reactingas an oxidizing agent. The oxygen source employed in accordance withthis pretreatment is preferably selected from the group consisting ofair, oxygen rich gas, ozone containing gas and hydrogen peroxide.“Oxygen rich gas” means pure oxygen and gas mixtures containing greaterthan about 21% oxygen by volume, preferably oxygen enriched air.Preferably, oxygen rich gas contains at least about 22% oxygen byvolume. “Ozone containing gas” means pure ozone and gas mixturescontaining ozone, preferably ozone enriched air. Preferably, the contentof ozone in the gas mixture is at least about 10 ppm ozone by volume.

For the practice of this pretreatment, one preferred oxygen source is anozone containing gas. Another preferred oxygen source for the practiceof this pretreatment is hydrogen peroxide. For providing the presence ofthe oxygen source in the dispersion during exposure to ultravioletlight, air, oxygen rich gas or ozone containing gas can be injectedcontinuously or intermittently into the dispersion, preferably instoichiometric excess, to provide the oxygen source during the exposureto ultraviolet light. Hydrogen peroxide can be added to the dispersion,also preferably in stoichiometric excess, by adding hydrogen peroxidesolution. The concentration of hydrogen peroxide is preferably about 0.1weight % to about 10 weight % based on fluoropolymer solids in thedispersion.

Ultraviolet light with an oxygen source is effective at ambient ormoderate temperatures and thus elevated temperatures are typically notrequired for the practice of this pretreatment. In a preferred form ofthis pretreatment, exposing the aqueous fluoropolymer dispersion toultraviolet light in the presence of an oxygen source is carried out ata temperature of about 5° C. to about 70° C., preferably about 15° C. toabout 70° C.

The time for carrying out this pretreatment with vary with factorsincluding the power of the ultraviolet light used, the type of oxygensource, processing conditions, etc. Preferred times for thispretreatment are about 15 minutes to about 10 hours.

Another preferred pretreatment comprises exposing the aqueousfluoropolymer dispersion to light having a wavelength of 10 nm to 760 nmin the presence of an oxygen source and photocatalyst. For the practiceof this pretreatment, the aqueous fluoropolymer dispersion is preferablyfirst diluted with water to a concentration less than the concentrationof the as polymerized aqueous fluoropolymer dispersion because,depending upon the equipment used, exposure to light can be moreeffective for reducing discoloration for dilute dispersions. Preferredconcentrations are about 2 weight percent to about 30 weight %, morepreferably about 2 weight percent to about 20 weight percent.

Light to be employed in accordance with this pretreatment has awavelength range or about 10 nm to about 760 nm. This wavelength rangeincludes ultraviolet light having a wavelength range of about 10 nm toabout 400 nm. Ultraviolet light has a wavelength range or about 10 nm toabout 400 nm and has been described to have bands including: UVA (315 nmto 400 nm), UVB (280 nm to 315 nm), and UVC (100 nm to 280 nm). Light tobe employed in accordance with this pretreatment also includes visiblelight having a wavelength range of about 400 nm to about 760 nm.

Any of various types of lamps can be used as the source of light. Forexample, submersible UV clarifier/sterilizer units sold for the purposesof controlling algae and bacterial growth in ponds are commerciallyavailable and may be used for the practice of this pretreatment. Theseunits include a low-pressure mercury-vapor UVC lamp within a housing forthe circulation of water. The lamp is protected by a quartz tube so thatwater can be circulated within the housing for exposure to ultravioletlight. Submersible UV clarifier/sterilizer units of this type are sold,for example, under the brand name Pondmaster by Danner Manufacturing,Inc. of Islandia N.Y. For continuous treatment processes, the dispersioncan be circulated though units of this type to expose the dispersion tolight. Single pass or multiple pass treatments can be employed.

Dispersion can also be processed in a batch operation in a containersuitable for exposure to light in the presence of an oxygen source andphotocatalyst. In this pretreatment, it is desirable for a suitablyprotected lamp to be immersed in the dispersion. For example, a vesselnormally used for coagulation of the aqueous fluoropolymer dispersion toproduce fluoropolymer resin can be used for carrying out the process ofthis pretreatment by immersing the lamp in the dispersion held in thisvessel. The dispersion can be circulated or stirred if desired tofacilitate exposure to the light. When the oxygen source is a gas asdiscussed below, circulation may be achieved or enhanced by injectingthe oxygen source into the dispersion. Ultraviolet lamps with protectivequartz tubes of the type employed in the submersible UVclarifier/sterilizer units can be employed for immersion in dispersionafter being removed from their housing. Other ultraviolet lamps such asmedium-pressure mercury vapor lamps can also be used with the lampsuitably protected for immersion in the dispersion such as by enclosingthe lamp in a quartz photowell. A borosilicate glass photowell can alsobe used although it may decrease effectiveness by filtering ultravioletlight in the UVC and UVB bands. Suitable medium-pressure mercury vaporlamps are sold by Hanovia of Fairfield, N.J.

As used in this pretreatment, “oxygen source” means any chemical sourceof available oxygen. “Available oxygen” means oxygen capable of reactingas an oxidizing agent. The oxygen source employed in accordance with thepresent this pretreatment is preferably selected from the groupconsisting of air, oxygen rich gas, ozone containing gas and hydrogenperoxide. “Oxygen rich gas” means pure oxygen and gas mixturescontaining greater than about 21% oxygen by volume, preferably oxygenenriched air. Preferably, oxygen rich gas contains at least about 22%oxygen by volume. “Ozone containing gas” means pure ozone and gasmixtures containing ozone, preferably ozone enriched air. Preferably,the content of ozone in the gas mixture is at least about 10 ppm ozoneby volume.

For the practice of this pretreatment, one preferred oxygen source is anozone containing gas. Another preferred oxygen source for the practiceof this pretreatment is hydrogen peroxide. For providing the presence ofthe oxygen source in the dispersion during exposure to ultravioletlight, air, oxygen rich gas or ozone containing gas can be injectedcontinuously or intermittently into the dispersion, preferably instoichiometric excess, to provide the oxygen source during the exposureto light. Hydrogen peroxide can be added to the dispersion, alsopreferably in stoichiometric excess, by adding hydrogen peroxidesolution. The concentration of hydrogen peroxide is preferably about 0.1weight % to about 10 weight % based on fluoropolymer solids in thedispersion.

Any of a variety of photocatalysts may be used in the practice of thispretreatment. Preferably, the photocatalyst is a heterogeneousphotocatalyst. Most preferably, the heterogeneous photocatalyst isselected from form the group consisting of titanium dioxide and zincoxide. For example, titanium dioxide sold under the tradename DegussaP25 having a primary particle size of 21 nm and being a mixture of 70%anatase and 30% rutile titanium dioxide has been found to be aneffective heterogeneous photocatalyst. Heterogeneous photocatalyst canbe used by dispersing it into the dispersion prior to exposure to light.Preferred levels of heterogenous photocatalyst are about 1 ppm to about100 ppm based on fluoropolymer solids in the dispersion.

Light with an oxygen source and photocatalyst is effective at ambient ormoderate temperatures and thus elevated temperatures are typically notrequired for the practice of this pretreatment. In a preferred processin accordance with this pretreatment, exposing the aqueous fluoropolymerdispersion to ultraviolet light in the presence of an oxygen source iscarried out at a temperature of about 5° C. to about 70° C., preferablyabout 15° C. to about 70° C.

The time for carrying out this pretreatment will vary with factorsincluding the power of the ultraviolet light used, the type of oxygensource, processing conditions, etc. Preferred times for thispretreatment are about 15 minutes to about 10 hours.

Another preferred pretreatment comprises exposing the aqueousfluoropolymer dispersion to hydrogen peroxide. For the practice of thispretreatment, the aqueous fluoropolymer dispersion is preferably firstdiluted with water to a concentration less than the concentration of theas polymerized aqueous fluoropolymer dispersion. Preferredconcentrations are about 2 weight percent to about 30 weight percent,more preferably about 2 weight percent to about 20 weight percent.

Exposing of the aqueous fluoropolymer dispersion to hydrogen peroxide ispreferably carried out by adding hydrogen peroxide to said aqueousfluoropolymer dispersion, preferably in an amount of about 0.1 weight %to about 10 weight percent based on weight of fluoropolymer solids.Preferably, the exposing of the aqueous fluoropolymer dispersion tohydrogen peroxide is carried out at a temperature of about 10° C. toabout 70° C., preferably about 25° C. to about 60° C. The time employedfor the exposure of the aqueous fluoropolymer dispersion is preferablyabout 1 hour to about 48 hours.

It is preferable for the practice of this pretreatment to also injectair, oxygen rich gas, or ozone containing gas into said fluoropolymerdispersion during the exposing of the aqueous fluoropolymer dispersionto the hydrogen peroxide. “Oxygen rich gas” means pure oxygen and gasmixtures containing greater than about 21% oxygen by volume, preferablyoxygen enriched air. Preferably, oxygen rich gas contains at least about22% oxygen by volume. “Ozone containing gas” means pure ozone and gasmixtures containing ozone, preferably ozone enriched air. Preferably,the content of ozone in the gas mixture is at least about 10 ppm ozoneby volume. Introduction of such gases can be accomplished by injectingthe gases into the aqueous fluoropolymer dispersion.

Preferably, the exposing of the aqueous fluoropolymer dispersion tohydrogen peroxide is carried out in the presence of Fe⁺², Cu⁺¹, or Mn⁺²ions. Preferably, the amount of Fe⁺², Cu⁺¹, or Mn⁺² ions is about 0.1ppm to about 100 ppm based on fluoropolymer solids in the dispersion.

Although the process can also be carried out in a continuous process,batch processes are preferable since batch processes facilitatecontrolled times for exposure of the hydrogen peroxide with the aqueousfluoropolymer dispersion to achieve the desired reduction in thermallyinduced discoloration. A batch process can be carried out in anysuitable tank or vessel of appropriate materials of construction and, ifdesired, has heating capability to heat the dispersion during treatment.For example, a batch process can be carried out in a vessel normallyused for coagulation of the aqueous fluoropolymer dispersion whichtypically includes an impeller which can be used to stirring thedispersion during treatment. Injection of air, oxygen rich gas, or ozonecontaining gas can also be employed to impart agitation to thedispersion.

Another preferred pretreatment comprises exposing the aqueousfluoropolymer dispersion to oxidizing agent selected from the groupconsisting of hypochlorite salts and nitrite salts. For the practice ofthis pretreatment, the aqueous fluoropolymer dispersion is preferablyfirst diluted with water to a concentration less than the concentrationof the as polymerized aqueous fluoropolymer dispersion. Preferredconcentrations are about 2 weight percent to about 30 weight percent,more preferably about 2 weight percent to about 20 weight percent.

Exposing of the aqueous fluoropolymer dispersion to oxidizing agentselected from the group consisting of hypochlorite salts and nitritesalts is preferably carried out by adding the oxidizing agent to theaqueous fluoropolymer dispersion, preferably in an amount of about 0.05weight % to about 5 weight percent based on weight of fluoropolymersolids. Preferred hypochlorite salts for addition to the dispersion aresodium hypochlorite or potassium hypochlorite. Sodium hypochlorite orpotassium hypochlorite are preferably used in an amount of about 0.05weight % to about 5 weight percent based on weight of fluoropolymersolids. Provided that aqueous medium of the dispersion is sufficientlyalkaline such as by containing sodium hydroxide, hypochlorite can alsobe generated in situ by injecting chlorine gas into the dispersion.Preferred nitrite salts for addition to the dispersion are sodiumnitrite, potassium nitrite and ammonium nitrite. Sodium nitrite,potassium nitrite and ammonium nitrite are preferably used in an amountof about 0.5 weight % to about 5 weight percent based on weight offluoropolymer solids.

Preferably, the exposing of the aqueous fluoropolymer dispersion to theoxidizing agent is carried out at a temperature of about 10° C. to about70° C. The exposure time with the aqueous fluoropolymer dispersion ispreferably about 5 minutes to about 3 hours.

It is preferable for the practice of this pretreatment to also introduceair, oxygen rich gas, or ozone containing gas into said fluoropolymerdispersion during the exposing of the aqueous fluoropolymer dispersionto the oxidizing agent. “Oxygen rich gas” means pure oxygen and gasmixtures containing greater than about 21% oxygen by volume, preferablyoxygen enriched air. Preferably, oxygen rich gas contains at least about22% oxygen by volume. “Ozone containing gas” means pure ozone and gasmixtures containing ozone, preferably ozone enriched air. Preferably,the content of ozone in the gas mixture is at least about 10 ppm ozoneby volume. Introduction of such gases can be accomplished by injectingsuch gases into the aqueous fluoropolymer dispersion.

Although the pretreatment can also be carried out in a continuousprocess, batch processes are preferable since batch processes facilitatecontrolled times for exposure of the hypochlorite salt or nitrite saltwith the aqueous fluoropolymer dispersion to achieve the desiredreduction in thermally induced discoloration. A batch process can becarried out in any suitable tank or vessel of appropriate materials ofconstruction and, if desired, has heating capability to heat thedispersion during treatment. For example, a batch process can be carriedout in a vessel normally used for coagulation of the aqueousfluoropolymer dispersion which typically includes an impeller which canbe used to stirring the dispersion during treatment. Injection of air,oxygen rich gas, or ozone containing gas can also be employed to impartagitation to the dispersion.

Another preferred pretreatment comprises adjusting the pH of the aqueousmedium of the aqueous fluoropolymer dispersion to greater than about 8.5and exposing the aqueous fluoropolymer dispersion to an oxygen source.For the practice of this pretreatment, the aqueous fluoropolymerdispersion is preferably first diluted with water to a concentrationless than the concentration of the as polymerized aqueous fluoropolymerdispersion. Preferred concentrations are about 2 weight percent to about30 weight percent, more preferably about 2 weight percent to about 20weight percent.

The pH of the aqueous fluoropolymer dispersion preferably is adjusted toabout 8.5 to about 11. More preferably, the pH of the aqueous medium ofthe aqueous fluoropolymer dispersion is adjusted to about 9.5 to about10.

The pH can be adjusted for the practice of this pretreatment by additionof a base which is sufficiently strong to adjust the pH of the aqueousfluoropolymer dispersion to the desired level and which is otherwisecompatible with the processing of the dispersion and the end useproperties of the fluoropolymer resin produced. Preferred bases arealkali metal hydroxides such as sodium hydroxide or potassium hydroxide.Ammonium hydroxide can also be used.

As used for this pretreatment, “oxygen source” means any chemical sourceof available oxygen. “Available oxygen” means oxygen capable of reactingas an oxidizing agent. The oxygen source employed in accordance withthis pretreatment is preferably selected from the group consisting ofair, oxygen rich gas, ozone containing gas and hydrogen peroxide.“Oxygen rich gas” means pure oxygen and gas mixtures containing greaterthan about 21% oxygen by volume, preferably oxygen enriched air.Preferably, oxygen rich gas contains at least about 22% oxygen byvolume. “Ozone containing gas” means pure ozone and gas mixturescontaining ozone, preferably ozone enriched air. Preferably, the contentof ozone in the gas mixture is at least about 10 ppm ozone by volume.

For the practice of this pretreatment, one preferred oxygen source is anozone containing gas. Another preferred oxygen source is for thepractice of this pretreatment is hydrogen peroxide. For providing theexposure of dispersion to the oxygen source, air, oxygen rich gas orozone containing gas can be injected continuously or intermittently intothe dispersion, preferably in stoichiometric excess. Hydrogen peroxidecan be added to the dispersion, also preferably in stoichiometricexcess, by adding hydrogen peroxide solution. The concentration ofhydrogen peroxide is preferably about 0.1 weight % to about 10 weight %based on fluoropolymer solids in the dispersion.

Preferably, the exposing of the aqueous fluoropolymer dispersion tooxygen source is carried out at a temperature of about 10° C. to about95° C., more preferably about 20° C. to about 80° C., most preferablyabout 25° C. to about 70° C. The time employed for the exposure of theaqueous fluoropolymer dispersion to oxygen source is preferably about 5minutes to about 24 hours.

Although the process can also be carried out in a continuous process,batch processes are preferable since batch processes facilitatecontrolled times for exposure of the hydrogen peroxide with the aqueousfluoropolymer dispersion to achieve the desired reduction in thermallyinduced discoloration. A batch process can be carried out in anysuitable tank or vessel of appropriate materials of construction and, ifdesired, has heating capability to heat the dispersion during treatment.For example, a batch process can be carried out in a vessel normallyused for coagulation of the aqueous fluoropolymer dispersion whichtypically includes an impeller which can be used to stirring thedispersion during treatment. Injection of air, oxygen rich gas, or ozonecontaining gas can also be employed to impart agitation to thedispersion.

More than one pretreatment can be used if desired.

The process of the present invention is useful for fluoropolymer resinwhich exhibits thermally induced discoloration which may range from mildto severe. The process is especially useful for aqueous fluoropolymerdispersion which contains hydrocarbon surfactant which causes thethermally induced discoloration, preferably aqueous fluoropolymerdispersion that is polymerized in the presence of hydrocarbonsurfactant.

The process of the invention is especially useful when the fluoropolymerresin prior to treatment exhibits significant thermally induceddiscoloration compared to equivalent commercial fluoropolymers. Theinvention is advantageously employed when the fluoropolymer resin has aninitial thermally induced discoloration value (L*_(i)) at least about 4L units below the L* value of equivalent fluoropolymer resin ofcommercial quality manufactured using ammonium perfluorooctanoatefluorosurfactant. The invention is more advantageously employed when theL*_(i) value is at least about 5 units below the L* value of suchequivalent fluoropolymer resin, even more advantageously employed whenthe L*_(i) value is at least 8 units below the L* value of suchequivalent fluoropolymer resin, still more advantageously employed whenthe L*_(i) value is at least 12 units below the L* value of suchequivalent fluoropolymer resin, and most advantageously employed whenthe L*_(i) value is at least 20 units below the L* value of suchequivalent fluoropolymer resin.

After the fluoropolymer resin is treated in accordance with the processof the invention, the resulting fluoropolymer resin is suitable for enduse applications appropriate for the particular type of fluoropolymerresin. Fluoropolymer resin produced by employing the present inventionexhibits reduced thermally induced discoloration without detrimentaleffects on end use properties.

Test Methods

Raw Dispersion Particle Size (RDPS) of polymer particles is measuredusing a Zetasizer Nano-S series dynamic light scattering systemmanufactured by Malvern Instruments of Malvern, Worcestershire, UnitedKingdom. Samples for analysis are diluted to levels recommended by themanufacturer in 10×10×45 mm polystyrene disposable cuvettes usingdeionized water that has been rendered substantially free of particlesby passing it through a sub-micron filter. The sample is placed in theZetasizer for determination of Dv(50). Dv(50) is the median particlesize based on volumetric particle size distribution, i.e. the particlesize below which 50% of the volume of the population resides.

The melting point (T_(m)) of melt-processible fluoropolymers is measuredby Differential Scanning calorimeter (DSC) according to the procedure ofASTM D 4591-07 with the melting temperature reported being the peaktemperature of the endotherm of the second melting. For PTFEhomopolymer, the melting point is also determined by DSC. The unmeltedPTFE homopolymer is first heated from room temperature to 380° C. at aheating rate of 10° C. and the melting temperature reported is the peaktemperature of the endotherm on first melting.

Comonomer content is measured using a Fourier Transform Infrared (FTIR)spectrometer according to the method disclosed in U.S. Pat. No.4,743,658, col. 5, lines 9-23 with the following modifications. The filmis quenched in a hydraulic press maintained at ambient conditions. Thecomonomer content is calculated from the ratio of the appropriate peakto the fluoropolymer thickness band at 2428 cm⁻¹ calibrated using aminimum of three other films from resins analyzed by fluorine 19 NMR toestablish true comonomer content. For instance, the % HFP content isdetermined from the absorbance of the HFP band at 982 cm⁻¹, and the PEVEcontent is determined by the absorbance of the PEVE peak at 1090 cm⁻¹.

Melt flow rate (MFR) of the melt-processible fluoropolymers are measuredaccording to ASTM D 1238-10, modified as follows: The cylinder, orificeand piston tip are made of a corrosion-resistant alloy, Haynes Stellite19, made by Haynes Stellite Co. The 5.0 g sample is charged to the 9.53mm (0.375 inch) inside diameter cylinder, which is maintained at 372°C.±1° C., such as disclosed in ASTM D 2116-07 for FEP and ASTM D 3307-10for PFA. Five minutes after the sample is charged to the cylinder, it isextruded through a 2.10 mm (0.0825 inch) diameter, 8.00 mm (0.315 inch)long square-edge orifice under a load (piston plus weight) of 5000grams. Other fluoropolymers are measured according to ASTM D 1238-10 atthe conditions which are standard for the specific polymer.

Measurement of Thermally Induced Discoloration

1) Color Determination

The L* value of fluoropolymer resin samples is determined using theCIELAB color scale, details of which are published in CIE Publication15.2 (1986). CIE L*a*b* (CIELAB) is the color space specified by theInternational Commission on Illumination (French Commissioninternationale de l'éclairage). It describes all the colors visible tothe human eye. The three coordinates of CIELAB represent the lightnessof the color (L*), its position between red/magenta and green (a*), andits position between yellow and blue (b*).

2) PTFE Sample Preparation and Measurement

The following procedure is used to characterize the thermally induceddiscoloration of PTFE polymers including modified PTFE polymers. 4.0gram chips of compressed PTFE powder are formed using a Carver stainlesssteel pellet mold (part #2090-0) and a Carver manual hydraulic press(model 4350), both manufactured by Carver, Inc. of Wabash, Ind. In thebottom of the mold assembly is placed a 29 mm diameter disk of 0.1 mmthick Mylar film. 4 grams of dried PTFE powder are spread uniformlywithin the mold opening poured into the mold and distributed evenly. Asecond 29 mm disk is placed on top of the PTFE and the top plunger isplaced in the assembly. The mold assembly is placed in the press andpressure is gradually applied until 8.27 MPa (1200 psi) is attained. Thepressure is held for 30 seconds and then released. The chip mold isremoved from the press and the chip is removed from the mold. Mylarfilms are pealed from the chip before subsequent sintering. Typicallyfor each polymer sample, two chips are molded.

An electric furnace is heated is heated to 385° C. Chips to be sinteredare placed in 4 inch×5 inch (10.2 cm×12.7 cm) rectangular aluminum trayswhich are 2 inches (5.1 cm) in depth. The trays are placed in thefurnace for 10 minutes after which they are removed to ambienttemperature for cooling.

4 gm chips processed as described above are evaluated for color using aHunterLab Color Quest XE made by Hunter Associates Laboratory, Inc. ofReston, Va. The Color Quest XE sensor is standardized with the followingsettings, Mode: RSIN, Area View: Large and Port Size: 2.54 cm. Theinstrument is used to determine the L* value of fluoropolymer resinsamples using the CIELAB color scale.

For testing, the instrument is configured to use CIELAB scale with D65Illuminant and 10° Observer. The L* value reported by this colorimeteris used to represent developed color with L* of 100 indicating a perfectreflecting diffuser (white) and L* of 0 representing black.

An equivalent fluoropolymer resin of commercial quality manufacturedusing ammonium perfluorooctanoate fluorosurfactant is used as thestandard for color measurements. For the Examples in this applicationillustrating the invention for PTFE fluoropolymer, an equivalentcommercial quality PTFE product made using ammonium perfluorooctanoatefluorosurfactant as the dispersion polymerization surfactant is TEFLON®601A. Using the above measurement process, the resulting colormeasurement for TEFLON® 601A is L*_(Std-PTFE)=87.3

3) Melt-Processible Fluoropolymers Sample Preparation and Measurement

The following procedure is used to characterize discoloration ofmelt-processible fluoropolymers, such as FEP and PFA, upon heating. A10.16 cm (4.00 inch) by 10.16 cm (4.00 inch) opening is cut in themiddle of a 20.32 cm (8.00 inch) by 20.32 cm (8.00 inch) by 0.254 mm(0.010 inch) thick metal sheet to form a chase. The chase is placed on a20.32 cm (8.00 inch) by 20.32 cm (8.00 inch) by 1.59 mm ( 1/16 inch)thick molding plate and covered with Kapton® film that is slightlylarger than the chase. The polymer sample is prepared by reducing size,if necessary, to no larger than 1 mm thick and drying. 6.00 grams ofpolymer sample is spread uniformly within the mold opening. A secondpiece of Kapton® film that is slightly larger than the chase is placedon top of the sample and a second molding plate, which has the samedimensions as the first, is placed on top of the Kapton® film to form amold assembly. The mold assembly is placed in a P-H-I 20 ton hot pressmodel number SP-210C—X4A-21 manufactured by Pasadena HydraulicsIncorporated of El Monte, Calif. that is set at 350° C. The hot press isclosed so the plates are just contacting the mold assembly and held for5 minutes. The pressure on the hot press is then increased to 34.5 MPa(5,000 psi) and held for an additional 1 minute. The pressure on the hotpress is then increased from 34.5 MPa (5,000 psi) to 137.9 MPa (20,000psi) over the time span of 10 seconds and held for an additional 50seconds after reaching 137.9 MPa (20,000 psi). The mold assembly isremoved from the hot press, placed between the blocks of a P-H-I 20 tonhot press model number P-210H manufactured by Pasadena HydraulicsIncorporated that is maintained at ambient temperature, the pressure isincreased to 137.9 MPa (20,000 psi), and the mold assembly is left inplace for 5 minutes to cool. The mold assembly is then removed from theambient temperature press, and the sample film is removed from the moldassembly. Bubble-free areas of the sample film are selected and 2.86 cm(1⅛ inch) circles are stamped out using a 1-⅛ inch arch punchmanufactured by C. S. Osborne and Company of Harrison, N.J. Six of thefilm circles, each of which has a nominal thickness of 0.254 mm (0.010inch) and nominal weight of 0.37 gram are assembled on top of each otherto create a stack with a combined weight of 2.2+/−0.1 gram.

The film stack is placed in a HunterLab ColorFlex spectrophotometer madeby Hunter Associates Laboratory, Inc. of Reston, Va., and the L* ismeasured using a 2.54 cm (1.00 inch) aperture and the CIELAB scale withD65 Illuminant and 10° Observer.

An equivalent fluoropolymer resin of commercial quality manufacturedusing ammonium perfluorooctanoate fluorosurfactant is used as thestandard for color measurements. For the Examples in this applicationillustrating the invention for FEP fluoropolymer resin, an equivalentcommercial quality FEP resin made using ammonium perfluorooctanoatefluorosurfactant as the dispersion polymerization surfactant is DuPontTEFLON® 6100 FEP. Using the above measurement process, the resultingcolor measurement for DuPont TEFLON® 6100 FEP is L*_(Std-FEP)=79.7.

4) % change in L* with respect to the standard is used to characterizethe change in thermally induced discoloration of the fluoropolymer resinafter treatment as defined by the following equation% change in L*=(L* _(t) −L* _(i))/(L* _(Std) −L* _(i))×100

-   L*_(i)=Initial thermally induced discoloration value, the measured    value for L on the CIELAB scale for fluoropolymer resins prior to    treatment to reduce thermally induced discoloration measured using    the disclosed test method for the type of fluoropolymer.-   L*_(t)=Treated thermally induced discoloration value, the measured    value for L on the CIELAB scale for fluoropolymer resins after    treatment to reduce thermally induced discoloration measured using    the disclosed test method for the type of fluoropolymer.-   Standard for PTFE: measured L*_(Std-PTFE)=87.3-   Standard for FEP: measured L*_(Std-FEP)=79.7

EXAMPLES

Apparatus for Drying of PTFE Polymer

A laboratory dryer for simulating commercially dried PTFE Fine Powder isconstructed as follows: A length of 4 inch (10.16 cm) stainless steelpipe is threaded on one end and affixed with a standard stainless steelpipe cap. In the center of the pipe cap is drilled a 1.75 inch (4.45 cm)hole through which heat and air source is introduced. A standard 4 inch(10.16 cm) pipe coupling is sawed in half along the radial axis and thesawed end of one piece is butt welded to the end of the pipe, oppositethe pipe cap. Overall length of this assembly is approximately 30 inches(76.2 cm) and the assembly is mounted in the vertical position with thepipe cap at the top. For addition of a control thermocouple, the 4 inchpipe assembly is drilled and tapped for a ¼ inch (6.35 mm) pipe fittingat a position 1.75 inch (4.45 cm) above the bottom of the assembly. A ¼inch (6.35 mm) male pipe thread to ⅛ inch (3.175 mm) Swagelok fitting isthreaded into the assembly and drilled through to allow the tip of a ⅛inch (3.175 mm) J-type thermocouple to be extended through the fittingand held in place at the pipe's radial center. For addition of a othergases, the 4 inch (10.16 cm) pipe assembly is drilled and tapped for a ¼inch (6.35 mm) pipe fitting at a position 180° from the thermocoupleport and higher at 3.75 inch (9.5 cm) above the bottom of the assembly.A ¼ inch (6.35 mm) male pipe thread to ¼ inch (6.35 mm) Swagelok fittingis threaded into the assembly and drilled through to allow the open endof a ¼ inch (6.35 mm) stainless steel tube to be extended through thefitting and held in place at the pipe's radial center. The entire pipeassembly is wrapped with heat resistant insulation that can easilywithstand 200° C. continuous duty.

The dryer bed assembly for supporting polymer is constructed as follows:A 4 inch (10.16 cm) stainless steel pipe nipple is sawed in half alongthe radial axis and onto the sawed end of one piece is tack weldedstainless steel screen with 1.3 mm wire size and 2.1 mm square opening.Filter media of polyether ether ketone (PEEK) or Nylon 6,6 fabric is cutinto a 4 inch (10.16 cm) disk and placed on the screen base. A 4 inch(10.16 cm) disk of stainless steel screen is placed on top of the filterfabric to hold it securely in place. Fabrics used include a Nylon 6,6fabric and PEEK fabric having the characteristics described in U.S. Pat.No. 5,391,709.

In operation, approximately ¼ inch (6.35 mm) of polymer is placeduniformly across the filter bed and the dryer bed assembly is screwedinto the bottom of the pipe assembly.

The heat and air source for this drying apparatus is a Master heat gun,model HG-751B, manufactured by Master Appliance Corp. of Racine, Wis.The end of this heat gun can be snuggly introduced through and supportedby the hole in the cap at the top of the pipe assembly. Control of airflow is managed by adjusting a damper on the air intake of the heat gun.Control of temperature is maintained by an ECS Model 800-377 controller,manufactured by Electronic Control Systems, Inc of Fairmont W. Va.Adaptation of the controller to the heat gun is made as follows: Thedouble pole power switch of the heat gun is removed. All power to theheat gun is routed through the ECS controller. The blower power issupplied directly from the ECS controller on/off switch. The heatercircuit is connected directly to the ECS controller output. Thethermocouple on the pipe assembly which is positioned above the polymerbed serves as the controller measurement device.

The apparatus described above is typically used to dry PTFE Fine Powderat 170° C. and can easily maintain that temperature to within ±1° C.

Apparatus for Drying of FEP Polymer

Equipment similar in design to that described in Apparatus for Drying ofPTFE Polymer is used except the scale is increased so the dryer bedassembly is 8 inch (20.32 cm) in diameter and the stainless steel screenis a USA standard testing sieve number 20 mesh. Unless otherwise noted,the apparatus is used to dry FEP for two hours with 180° C. air and caneasily maintain that temperature to within ±1° C. Typical polymerloading is 18 grams dry weight of polymer.

A secondary dryer bed assembly is produced by the addition of threeevenly spaced nozzles with a centerline 3.0 cm above the polymer bed.The nozzles can be used to introduce additional gasses to the dryingair. One of many possible configurations is to connect an AQUA-6portable ozone generator manufactured by A2Z Ozone of Louisville, Ky. toeach of the nozzles.

10 Watt UVC Light Source

For experiments using 10 watt UVC light sources, the 254 nm lamps areobtained from 10 watt Pondmaster® submersible UV clarifier/sterilizerunits manufactured by Danner Manufacturing, Inc. of Islandia, N.Y. Theseunits, commonly used in the aquaculture industry, consist of 4 majorcomponents: (1) A ballast which provides the proper power supply. (2) Alow pressure mercury vapor lamp which emits UVC radiation uponactivation. (3) A quartz tube which protects the lamp and electronicsfrom water damage while allowing short wavelength UV light to pass. (4)A dark plastic outer housing which is threaded at one end so as to screwonto the ballast and provide a seal around the quartz tube, therebyprotecting lamp and electronics from water penetration. The housing isalso designed to allow water to flow from one end of the protected lampto the other end while preventing hazardous UV light from escaping thehousing. For purposes of this experimentation, the plastic housing isremoved and the threaded end is removed by saw. The treaded adapter isthen screwed back into the ballast, thereby sealing the quartz tube tothe ballast but eliminating the black plastic UV shield. In this way thelight source is made useful for batch (ie. non flow through)experiments.

Light intensity is measured with a meter that has the capability ofreading up to 20.0 milliwatts/cm² (mW/cm²) by positioning three sensors(245 nm UVC, 310 nm UVB and 365 nm UVA) four inches from the quartzprotective tube. Measurements: UVC is 1.06 mW/cm², UVB is 33.7microwatt/cm² and UVA is 19.2 microwatt/cm².

450 Watt Hanovia Lamp Light Source

For experiments using a 450 watt Hanovia lamp, a Model PC451.050 450watt medium pressure mercury vapor lamp, manufactured by Hanovia, Inc.of Fairfield, N.J. is used with the following setup: An Ace GlassIncorporated, Model 6386-20, 2000 ml jacketed filter reactor body isfitted with an Ace Glass, Inc. Model 5846-60 bottom PTFE plug in which arecess is machined to support a 48 mm diameter, jacketed immersionphotowell. The photowell is connected to a circulating cooling bath ofsufficient capacity to keep the coolant temperature exiting thephotowell below 40° C. The lamp is operated with an appropriatelymatched power supply such as the Ace Glass Model No. 7830-58. A quartzphotowell (Ace Glass Part #7874-23) or a borosilicate photowell (AceGlass Part #7875-30) may be used although it may decrease effectivenessby filtering some ultraviolet light in the UVC and UVB bands.

Light intensity is measured with a meter (UVP Model UVX Radiometer) thathas the capability of reading up to 20.0 milliwatts/cm² (mW/cm²) bypositioning three sensors (245 nm UVC (UVP Model UVX-25), 310 nm UVB(UVP Model UVX-31) and 365 nm UVA (UVP Model UVX36)) 3.5 inches from theborosilicate well. When the Hanovia 450 watt lamp is fully heated up,the UVC reads 10.11 mW/cm², the UVB reads 9.37 mW/cm² and the UVA reads17.0 mW/cm².

When similar measurement is made with the quartz photowell, even beforethe Hanovia 450 watt lamp is fully heated up, the light intensity is sostrong as to reach the maximum measurement capability of the light meterused.

Fluoropolymer Preparation

PTFE—Preparation of Hydrocarbon Stabilized PTFE Dispersion

To a 12 liter, horizontally disposed, jacketed, stainless steelautoclave with a two blade agitator is added 5200 gm of deionized,deaerated water. To the autoclave is added an additional 500 gm ofdeionized, deaerated water which contains 0.12 gm of Pluronic® 31R1. Theautoclave is sealed and placed under vacuum. The autoclave pressure israised to 30 psig (308 kPa) with nitrogen and vented to atmosphericpressure. The autoclave is pressured with nitrogen and vented 2 moretimes. Autoclave agitator speed is set at 65 RPM. 20 ml of initiatorsolution containing 1.0 gm of ammonium persulfate (APS) per liter ofdeionized, deaerated water is added to the autoclave.

The autoclave is heated to 90° C. and TFE is charged to the autoclave tobring the autoclave pressure to 400 psig (2.86 MPa). 150 ml of aninitiator solution composed of 11.67 gm of 70% active disuccinic acidperoxide (DSP), 0.167 gm of APS and 488.3 gm of deionized water ischarged to the autoclave at 80 ml/min. After the autoclave pressuredrops 10 psi (69 kPa) from the maximum pressure observed duringinjection of initiator solution, the autoclave pressure is brought backto 400 psig (2.86 MPa) with TFE and maintained at that pressure for theduration of the polymerization. After 100 gm of TFE has been fed sincekickoff, an aqueous surfactant solution containing 5733 ppm of SDShydrocarbon stabilizing surfactant and 216 ppm of iron sulfateheptahydrate is pumped to the autoclave at a rate of 4 ml/min until 185ml of surfactant solution has been added. After approximately 70 minutessince kickoff, 1500 gm of TFE has been added to the autoclave. Theagitator is stopped, the autoclave is vented to atmospheric pressure andthe dispersion is cooled and discharged. Solids content of thedispersion is 18-19 wt %. Dv(50) raw dispersion particle size (RDPS) is208 nm.

Isolation of PTFE Dispersion

To a clean glass resin kettle having internal dimensions 17 cm deep and13 cm in diameter is charged 600 gm of 5 wt % dispersion. The dispersionis agitated with a variable speed, IKA Works, Inc., RW20 digitaloverhead stirrer affixed with a 6.9 cm diameter, rounded edge threeblade impeller having a 45° downward pumping pitch. The followingsequence is executed until the dispersion has completely coagulated asindicated by the separation of white PTFE polymer from a clear aqueousphase: At time zero, agitation speed is set at 265 revolutions perminute (RPM) and 20 ml of a 20 wt % aqueous solution of ammoniumcarbonate is slowly added to the resin kettle. At 1 minute from timezero, the agitator speed is raised to 565 RPM and maintained until thedispersion is completely coagulated. Once coagulated, the clear aqueousphase is removed by suction and 600 ml of cold (approximately 6° C.),deionized water is added. The slurry is agitated at 240 RPM for 5minutes until agitation is halted and the wash water removed from theresin kettle. This washing procedure is repeated two more times with thefinal wash water being separated from the polymer by vacuum filtrationas indicated below.

A ceramic filtration funnel (10 cm internal diameter) is placed on avacuum flask with rubber sealing surface. A 30 cm by 30 cm lint freenylon filter cloth is placed in the filtration funnel and the washedpolymer and water is poured into the funnel. A vacuum is pulled on thevacuum flask and once the wash water is removed, 1200 ml of additionaldeionized water is poured over the polymer and pulled through thepolymer into the vacuum flask. Polymer thus coagulated, washed andisolated is removed from the filter cloth for further processing.

FEP: Preparation of Hydrocarbon Stabilized TFE/HFP/PEVE Dispersion

A cylindrical, horizontal, water-jacketed, paddle-stirred, stainlesssteel reactor having a length to diameter ratio of about 1.5 and a watercapacity of 10 gallons (37.9 L) is charged with 60 pounds (27.2 kg) ofdeionized water. The reactor temperature then is increased to 103° C.while agitating at 46 rpm. The agitator speed is reduced to 20 rpm andthe reactor is vented for 60 seconds. The reactor pressure is increasedto 15 psig (205 kPa) with nitrogen. The agitator speed is increased to46 rpm while cooling to 80° C. The agitator speed is reduced to 20 rpmand a vacuum is pulled to 12.7 psia (88 kPa). A solution containing 500ml of deaerated deionized water, 0.5 grams of Pluronic® 31R1 solutionand 0.3 g of sodium sulfite is drawn into the reactor. With the reactorpaddle agitated at 20 rpm, the reactor is heated to 80° C., evacuatedand purged three times with TFE. The agitator speed is increased to 46rpm and the reactor temperature then is increased to 103° C. After thetemperature has become steady at 103° C., HFP is added slowly to thereactor until the pressure is 430 psig (3.07 MPa). 112 ml of liquid PEVEis injected into the reactor. Then TFE is added to the reactor toachieve a final pressure of 630 psig (4.45 MPa). Then 80 ml of freshlyprepared aqueous initiator solution containing 2.20 wt % of ammoniumpersulfate (APS) is charged into the reactor. Then, this same initiatorsolution is pumped into the reactor at a TFE to initiator solution massratio of twenty-to-one for the remainder of the polymerization afterpolymerization has begun as indicated by a 10 psi (69 kPa) drop inreactor pressure, i.e. kickoff. Additional TFE is also added to thereactor beginning at kickoff at a rate of 0.06 lb/min (0.03 kg/min)subject to limitation in order to prevent the reactor from exceeding themaximum desired limit of 650 psig (4.58 MPa) until a total of 12.0 lb(5.44 kg) of TFE has been added to the reactor after kickoff.Furthermore, liquid PEVE is added to the reactor beginning at kickoff ata rate of 0.3 ml/min for the duration of the reaction.

After 4.0 lb (1.8 kg) of TFE has been fed since kickoff, an aqueoussurfactant solution containing 45,176 ppm of SDS hydrocarbon stabilizingsurfactant and 60,834 ppm of 30% ammonium hydroxide solution is pumpedto the autoclave at a rate of 0.2 ml/min. The aqueous surfactantsolution pumping rate is increased to 0.3 ml/min after 6.0 lb (2.7 kg)of TFE has been fed since kickoff, then to 0.4 ml/min after 8.0 lb (3.6kg) of TFE has been fed since kickoff, to 0.6 ml/min after 10.0 lb (4.5kg) of TFE has been fed since kickoff, and finally to 0.8 ml/min after11.0 lb (5.0 kg) of TFE has been fed since kickoff resulting in a totalof 47 ml of surfactant solution added during reaction. The totalreaction time is 201 minutes after initiation of polymerization duringwhich 12.0 lb (5.44 kg) of TFE and 60 ml of PEVE are added. At the endof the reaction period, the TFE feed, PEVE feed, the initiator feed andsurfactant solution feed are stopped; an additional 25 ml of surfactantsolution is added to the reactor, and the reactor is cooled whilemaintaining agitation. When the temperature of the reactor contentsreaches 90° C., the reactor is slowly vented. After venting to nearlyatmospheric pressure, the reactor is purged with nitrogen to removeresidual monomer. Upon further cooling, the dispersion is dischargedfrom the reactor at below 70° C.

Solids content of the dispersion is 20.07 wt % and Dv(50) raw dispersionparticle size (RDPS) is 143.2 nm. 703 grams of wet coagulum is recoveredon cleaning the autoclave. The TFE/HFP/PEVE terpolymer (FEP) has a meltflow rate (MFR) of 29.6 gm/10 min, an HFP content of 9.83 wt %, a PEVEcontent of 1.18 wt %, and a melting point of 256.1° C.

Isolation of FEP Dispersion

The dispersion is coagulated by freezing the dispersion at −30° C. for16 hours. The dispersion is thawed and the water is separated from thesolids by filtering through a 150 micron mesh filter bag modelNMO150P1SHS manufactured by The Strainrite Companies of Auburn, Me.

Thermally Induced Discoloration

Dried polymer is characterized as described above in the TestMethods—Measurement of Thermally Induced Discoloration as applicable tothe type of polymer used in the following Examples.

Comparative Example 1 PTFE with Hydrocarbon Stabilizing Surfactant—NoTreatment

A quantity of PTFE dispersion as described above is diluted to 5 wt %solids with deionized water. The dispersion is coagulated and isolatedvia the method described above (Isolation of PTFE Dispersion). Polymerthus obtained is then dried at 170° C. for 1 hour using the PTFE drierdescribed above in Apparatus for Drying of PTFE Polymer. Dried polymeris characterized for thermally induced discoloration as described in theTest Methods Measurement of Thermally Induced Discoloration for PTFE.Resulting value for L*_(i) is 43.9, indicating extreme discoloration ofthe polymer upon thermal processing for untreated polymer. The measuredcolor is shown in Table 1.

Example 1a PTFE, Dried with Ozone at ½ Power

A quantity of PTFE Dispersion as described above is diluted to 5 wt %solids with deionized water. The dispersion is coagulated and isolatedvia the method described above (Coagulation and Isolation of PTFEDispersion). Polymer thus obtained is then dried at 170° C. for 1 hourusing the PTFE drier described above (Apparatus for Drying of PTFEPolymer). During the hour of drying, 100 cc/min of ozone enriched air isintroduced into the dryer. Ozone is produced by passing 100 cc/min ofair into a Clearwater Technologies, Inc. Model CD-10 ozone generatorwhich is operated at ½ power setting. Dried polymer is characterized asdescribed in the Test Methods Measurement of Thermally InducedDiscoloration for PTFE. L* obtained for this polymer is 63.7 with achange in L* of 45.6% indicating a much improved color after treatment.The measured color is shown in Table 1.

Example 1b PTFE, Dried with Ozone at Full Power

Example 1 is repeated except the ozone generator is operated at fullpower. L* obtained for this polymer is 65.9 with a % change in L* of50.7% indicating a much improved color after treatment. The measuredcolor is shown in Table 1.

Comparative Example 2 PTFE, UVC, 1 wt % H₂O₂ on Polymer, O₂ Injection, 3Hours, 60° C.

To a glass beaker is added 155 gm of PTFE dispersion as prepared abovehaving 19.4% solids and 1.0 gm of 30 wt % hydrogen peroxide. The netweight is raised to 600 gm with deionized water, thus reducing the %solids to 5 wt %. A total of 1800 gms of dispersion thus prepared isadded to a 2000 ml jacketed resin kettle. The dispersion is heated to60° C. with agitation aided by continuous injection with 100 cc/min ofoxygen through two sintered glass, fine bubble, sparge tubes. Two 10watt 254 nm UV lights are immersed in the dispersion. The lights areenergized for 3 hours. 1200 gm of the treated dispersion is coagulatedand isolated as described above. Half of the resulting wet polymer isdried in the apparatus for drying of PTFE polymers at 170° C. for 1 hourusing only air as the drying gas. Dried polymer is characterized asdescribed in the Test Methods Measurement of Thermally InducedDiscoloration for PTFE. L* obtained for this polymer is 75.9 with a %change in L* of 73.7%. The measured color is shown in Table 1.

Example 2 PTFE, UVC, 1 wt % H₂O₂ on polymer, O₂ Injection, 3 Hours, 60°C.

The remaining half of wet polymer obtained from Comparative Example 2after coagulation and isolation is dried in the apparatus for drying ofPTFE polymers described above with the addition of ozone enriched air.During the hour of drying at 170° C., 100 cc/min of ozone enriched airis introduced into the dryer. Ozone is produced by passing 100 cc/min ofair into a Clearwater Technologies, Inc. Model CD-10 ozone generatorwhich is operated at the full power setting. Dried polymer ischaracterized for Thermally Induced Discoloration. L* obtained for thispolymer is 84.9 with a % change in L* of 94.5% indicating a muchimproved color after treatment. The measured color is shown in Table 1.

Comparative Example 3 PTFE, 0.33-0.5 wt % NaOCl on poly, 1 Hour, AmbientTemp

To a glass resin kettle is added 155 gm of PTFE dispersion as describedabove having 19.4% solids. The net weight is raised to 600 gm withdeionized water, thus reducing the % solids to 5 wt %. To the dispersionis added 1.0 gm of 10-15 wt % sodium hypochlorite solution. Thedispersion is agitated at 240 rpm for 1 hour with a variable speed, IKAWorks, Inc., RW20 digital overhead stirrer affixed with a 6.9 cmdiameter, rounded edge three blade impeller having a 45° downwardpumping pitch. The resulting, treated dispersion is coagulated andisolated as described above, dried in the apparatus for drying of PTFEpolymers using only ambient air as the drying gas and finallycharacterized for Thermally Induced Discoloration. L* obtained for thispolymer is 57.2 with a % change in L* of 30.6%. The measured color isshown in Table 1.

Example 3 PTFE, 0.33-0.5 wt % NaOCl on poly, 1 Hour, Ambient Temp

The procedure of Comparative Example 3 is repeated and after coagulationand isolation, the wet polymer is dried in the apparatus for drying ofPTFE polymers with the addition of ozone enriched air. During the hourof drying at 170° C., 100 cc/min of ozone enriched air is introducedinto the dryer. Ozone is produced by passing 100 cc/min of air into aClearwater Technologies, Inc. Model CD-10 ozone generator which isoperated at the full power setting. Dried polymer is characterized forThermally Induced Discoloration. L* obtained for this polymer is 84.9with a % change in L* of 94.5% indicating a much improved color aftertreatment. The measured color is shown in Table 1.

Comparative Example 4 PTFE, NaOH pH=9.9, Oxygen, 3.0 Hours @50° C.

To a 2000 ml jacketed resin kettle is added 465 gm of PTFE Dispersion asdescribed above having a solids content of 19.4 wt %. Net weight israised to 1800 gm with deionized water. While agitating at 300 rpm, thedispersion is heated to 50° C. by setting the appropriate temperature onthe jacket circulating bath. Once at temperature, pH of the dispersionis adjusted to 9.9 by adding approximately 8 drops of 50 wt % sodiumhydroxide solution to the resin kettle The dispersion is sparged withoxygen through a 25 mm diameter sintered glass, fine bubble, spargetube. Dispersion temperature is held constant and agitation is continuedfor 3.0 hours. 1200 gm of the treated dispersion is coagulated andisolated as described above. Half of the resulting wet polymer is driedat 170° C. for one hour in the apparatus for drying of PTFE polymersusing only air as the drying gas. Dried Polymer is characterized forThermally Induced Discoloration. L* obtained for this polymer is 54.2with a % change in L* of 23.7%. The measured color is shown in Table 1.

Example 4 PTFE, NaOH pH=9.9, Oxygen, 3.0 Hours @50° C.

The remaining half of wet polymer obtained from Comparative Example 4after coagulation and isolation is dried at 170° C. for 1 hour in theapparatus for drying of PTFE polymers with the addition of ozoneenriched air. During the hour of drying, 100 cc/min of ozone enrichedair is introduced into the dryer. Ozone is produced by passing 100cc/min of air into a Clearwater Technologies, Inc. Model CD-10 ozonegenerator which is operated at the full power setting. The dried polymeris characterized for Thermally Induced Discoloration. L* obtained forthis polymer is 81.3 with a % change in L* of 86.2% indicating a muchimproved color after treatment. The measured color is shown in Table 1.

Comparative Example 5 (PTFE, NaOH pH=9.9, Oxygen, 1.0 Hour @50° C.

To a 2000 ml jacketed resin kettle is added 310 gm of PTFE

Dispersion as described above having a solids content of 19.4 wt %. Netweight is raised to 1200 gm with deionized water. While agitating at 300rpm, the dispersion is heated to 50° C. by setting the appropriatetemperature on the jacket circulating bath. Once at temperature, pH ofthe dispersion is adjusted to 9.9 by adding approximately 5 drops of 50wt % sodium hydroxide solution to the resin kettle. The dispersion issparged with oxygen through a 25 mm diameter sintered glass, finebubble, sparge tube. Dispersion temperature is held constant andagitation is continued for 1.0 hour. The treated dispersion iscoagulated and isolated as described above. Half of the resulting wetpolymer is dried in the apparatus for drying of PTFE polymers at 170° C.for one hour using only air as the drying gas. Dried polymer ischaracterized for Thermally Induced Discoloration. L* obtained for thispolymer is 49.3 with a % change in L* of 12.4%. The measured color isshown in Table 1.

Example 5 PTFE, NaOH pH=9.9, Oxygen, 1.0 Hours @50° C.

The remaining half of wet polymer obtained from Comparative Example 5after coagulation and isolation is dried in the apparatus for drying ofPTFE polymers at 170° C. for one hour with the addition of ozoneenriched air. During the hour of drying, 100 cc/min of ozone enrichedair is introduced into the dryer. Ozone is produced by passing 100cc/min of air into a Clearwater Technologies, Inc. Model CD-10 ozonegenerator which is operated at the full power setting. Dried polymer ischaracterized for Thermally Induced Discoloration. L* obtained for thispolymer is 75.5 with a % change in L* of 72.8% indicating a muchimproved color after treatment. The measured color is shown in Table 1.

TABLE 1 L* L* drying with % change in L drying % change ozone with ozoneExamples with air in L* enriched air enriched air Comp Ex 1 (No 43.9Treatment) Example 1a — — 63.7 45.6% Example 1b — — 65.9 50.7% Comp Ex 275.9 73.7% — — Example 2 — — 84.9 94.5% Comp Ex 3 57.2 30.6% — — Example3 — — 84.9 94.5% Comp Ex 4 54.2 23.7% — — Example 4 — — 81.3 86.2% CompEx 5 49.3 12.4% — — Example 5 — — 75.5 72.8%

Comparative Example 6 FEP—No Treatment

Aqueous FEP dispersion polymerized as described above is diluted to 5weight percent solids with deionized water. The dispersion is coagulatedby freezing the dispersion at −30° C. for 16 hours. The dispersion isthawed and the water is separated from the solids by filtering through a150 micron mesh filter bag model NMO150P1SHS manufactured by TheStrainrite Companies of Auburn, Me. The solids are divided to allow thesample to be dried by more the one technique.

A first portion of polymer is dried for 2 hours with 180° C. air in theequipment described under Apparatus for Drying of FEP Polymer solidsusing only air as the drying gas. The dried powder is molded to producecolor films to characterize for thermally induced discoloration asdescribed in the Test Methods section above as Measurement of ThermallyInduced Discoloration for FEP. L* obtained for this polymer is 44.8. Themeasured color is shown in Table 2.

Example 7 FEP—Ozone Drying

Another portion of polymer prepared in Comparative Example 6 is driedfor 2 hours with 180° C. air that is enriched with ozone in theequipment described under Apparatus for Drying of FEP Polymer with thedryer bed assembly with three evenly spaced nozzles. Each nozzle isconnected to an AQUA-6 portable ozone generator manufactured by A2ZOzone of Louisville, Ky., which is operated during the drying process.L* obtained for this polymer is 55.8 with a % change in L* of 31.5%indicating a much improved color after treatment. The measured color isshown in Table 2.

Example 8 FEP—Pretreatment with UVC+Ozone Injection

Aqueous FEP dispersion polymerized as described Comparative Example 6 isdiluted to 5 weight percent solids with deionized water and preheated to40° C. in a water bath. A fresh FeSO₄ solution is prepared by diluting0.0150 g of FeSO₄-7H₂O to 100 ml using deaerated deionized water. 1200ml of the FEP dispersion, 4 ml of the FeSO₄ solution, and 2 ml of 30 wt% H₂O₂ are added to a 2000 ml jacketed glass reactor with internaldiameter of 10.4 cm, which has 40° C. water circulating through thereactor jacket, and the contents are mixed. Two sparge tubes that eachhave a 12 mm diameter by 24 mm long, fine-bubble, fritted-glass cylinderproduced by LabGlass as part number 8680-130 are placed in the reactor,and each is connected to an AQUA-6 portable ozone generator describedabove. The ozone generators are turned on and used to bubble 1.18standard L/min (2.5 standard ft³/hr) of ozone enriched air through thedispersion. The dispersion is allowed to equilibrate for 5 minutes. A 10watt UVC light as described in 10 watt UVC Light Source is placed in thereactor. The UVC lamp is turned on to illuminate the dispersion whileinjection with ozone enriched air and controlling temperature at 40° C.After three hours, the lamp is extinguished and the ozone enriched airis stopped. The dispersion is coagulated, filtered, dried and molded asdescribed in Comparative Example 6 to compare the differences betweendrying with air only and ozone enriched air. L* obtained for polymerdried with air only is 58.4 with a % change in L* of 39.0%. L* obtainedfor polymer dried with ozone enriched air is 76.2 with a % change in L*of 90.0% indicating a much improved color after treatment. The measuredcolor is shown in Table 2.

Example 9 FEP—Pretreatment with UVC+Oxygen Injection

Treatment is conducted utilizing the same conditions as Example 8 except1.0 standard L/min of oxygen is bubbled through a sparge tube with a 25mm diameter fine fritted glass disc sparge tube produced by Ace Glass aspart number 7196-20 in place of ozone. Dried polymer is characterizedfor Thermally Induced Discoloration.

L* obtained for polymer dried with air only is 55.2 with a % change inL* of 29.8%. L* obtained for polymer dried with ozone enriched air is60.4 with a % change in L* of 44.7% indicating a much improved colorafter treatment. The measured color is shown in Table 2.

Example 10 FEP—Pretreatment with H₂O₂ Treatment

Aqueous FEP dispersion polymerized as described in Comparative Example 6is diluted to 5 weight percent solids with deionized water. 1200 ml ofthe FEP dispersion preheated to 50° C. in a water bath. The preheateddispersion and 2 ml of 30 wt % H₂O₂ are added to a 2000 ml jacketedglass reactor with internal diameter of 13.3 cm (5-¼ inches) that has50° C. water circulating through the reactor jacket. An impeller withfour 3.18 cm (1.25 inch) long flat blades set at a 45° angle and twosparge tubes that each have a 12 mm diameter by 24 mm long fine-bubble,fritted-glass cylinder produced by LabGlass as part number 8680-130 areplaced in the reactor. The sparge tubes are connected to an air supplythat is passed through a Drierite gas purification column model 27068produced by W.A. Hammond Drierite Company of Xenia, Ohio, and the airsupply is adjusted to deliver 1.42 standard L/min (3.0 standard ft³/hr).The agitator is set at 60 rpm. After 5 minutes of mixing, the dispersiontemperature is 49.5° C. and the reaction timer is started. After 45minutes of reaction, 50 ml of deionized water and 2 ml of 30 wt % H₂O₂are added to offset evaporative losses. The reaction is ended after 16hours by stopping the agitator, ceasing the air flow, discontinuing thehot water circulation, and then removing the dispersion from thereactor. The dispersion is coagulated, filtered, dried and molded asdescribed in Comparative Example 6 to compare the differences betweendrying with air only and ozone enriched air. L* obtained for polymerdried with air only is 35.2 with a % change in L* of −27.5%. L* obtainedfor polymer dried with ozone enriched air is 63.7 with a % change in L*of 54.2% indicating a much improved color after treatment. The measuredcolor is shown in Table 2. It is to be noted that the pretreatment inthis example results in dried polymer in air alone showing a severedecrease in the value of L* as compared to untreated polymer. However,drying the pretreated polymer in ozone enriched air results in a greater% change in L* than polymer dried in ozone enriched air with nopretreatment (see Comparative Example 6 which shows a % change ofL*=31.5%). This shows that the pretreatment of dispersion with H₂O₂confers an added beneficial effect in improving the thermally induceddiscoloration when drying polymer with ozone enriched air.

Example 11 FEP—Pretreatment with UVC+Catalyst+Oxygen Injection

Aqueous FEP dispersion polymerized as described Comparative Example 6 isdiluted to 5 weight percent solids with deionized water and preheated to40° C. in a water bath. A TiO₂ solution is produced by sonicating 0.0030g of Degussa P-25 TiO₂ lot Kontrollnummer 1263 diluted to 6 ml withdeionized water. 1200 ml of the FEP dispersion, all 6 ml of the TiO₂solution, and 2 ml of 30 wt % H₂O₂ are added to a 2000 ml jacketed glassreactor with internal diameter of 10.4 cm, which has 40° C. watercirculating through the reactor jacket, and the contents are mixed. Asparge tube with a 25 mm diameter fine-bubble, fritted-glass disc spargetube produced by Ace Glass as part number 7196-20 is placed in thereactor, and 1.0 standard L/min of oxygen is bubbled through thedispersion. The dispersion is allowed to equilibrate for 5 minutes. A 10watt UVC light as described in 10 watt UVC Light Source is placed in thereactor. The UVC lamp is turned on to illuminate the dispersion whileinjection with oxygen and controlling temperature at 40° C. After threehours, the lamp is extinguished and the sparge gas is stopped. Thedispersion is coagulated, filtered, dried and molded as described inComparative Example 6 to compare the differences between drying with aironly and ozone enriched air. L* obtained for polymer dried with air onlyis 63.3 with a % change in L* of 53.0%. L* obtained for polymer driedwith ozone enriched air is 79.0 with a % change in L* of 98.0%indicating a much improved color after treatment. The measured color isshown in Table 2.

TABLE 2 L* drying % change in L L* drying with ozone with ozone Exampleswith air % change in L enriched air enriched air Comp Ex 6 44.8 — — —(No Treatment) Example 7 — — 55.8 31.5% Example 8 58.4 39.0% 76.2 90.0%Example 9 55.2 29.8% 60.4 44.7% Example 10 35.2 −27.5% 63.7 54.2%Example 11 63.3 53.0% 79.0 98.0%

Comparative Example 7 PTFE—No Treatment

An aqueous PTFE dispersion is made similar to the method described above(PTFE—Preparation of Hydrocarbon Stabilized PTFE Dispersion) but withaddition to the reactor of 4100 grams of SDS per million grams of drypolymer. The resulting dispersion is diluted to 15 wt % solids withdeionized water and is isolated as described above (Isolation of PTFEDispersion) with the exception that the 600 grams of dispersion is 15 wt% solids rather than the 5 wt % solids described above. Approximately 55gm of the wet polymer is then dried at 180° C. for 1 hour using the PTFEdrier described above (Apparatus for Drying of PTFE Polymer). Driedpolymer is characterized for thermally induced discoloration asdescribed in the Test Methods Measurement of Thermally InducedDiscoloration for PTFE. The measured L* value is 34.0 and is shown inTable 3.

Example 12 PTFE—Drying with Ozone Enriched Air

Approximately 55 gm of the wet polymer as produced in ComparativeExample 7—PTFE is dried at 180° C. for 1 hour using the PTFE drierdescribed above (Apparatus for Drying of PTFE Polymer) with the additionof ozone enriched air. Ozone is produced by passing 300 cc/min of airinto a ClearWater Tech. LLC Model CD-10 ozone generator which isoperated at the full power setting. Dried polymer is characterized forthermally induced discoloration as described in the Test MethodsMeasurement of Thermally Induced Discoloration for PTFE. The measured L*value and the % change in L* as calculated relative to ComparativeExample 7—PTFE are shown in Table 3.

Example 12 PTFE Demonstrates the Benefit of Employing Ozone Enriched Airas the Oxygen Source

TABLE 3 Effect of Ozone as Oxygen Source Example O3-Air cc/min L* % Chgin L* Comp. Ex. 7 - PTFE 0 34.0 Example 12 - PTFE 300 47.9 26.1

Example 13 PTFE—Drying with Ozone Enriched Air in the Presence of AlkaliMetal Salt, KCl

An aqueous PTFE dispersion is made similar to the method described above(PTFE—Preparation of Hydrocarbon Stabilized PTFE Dispersion) but withaddition to the reactor of 4100 grams of SDS per million grams of drypolymer. The resulting dispersion is diluted to 15 wt % solids withdeionized water. 0.145 gm of the alkali metal salt, KCl, is added to 600gm of the dilute dispersion and thoroughly mixed. The resulting mixtureis isolated as described above (Isolation of PTFE Dispersion) with theexception that the 600 grams of dispersion is 15 wt % solids rather thanthe 5 wt % solids described above. Approximately 55 gm of the wetpolymer is then dried at 180° C. for 1 hour using the PTFE drierdescribed above (Apparatus for Drying of PTFE Polymer) with the additionof ozone enriched air. Ozone is produced by passing 300 cc/min of airinto a Clearwater Technologies, Inc. Model CD-10 ozone generator whichis operated at the full power setting. Dried polymer is characterizedfor thermally induced discoloration as described in the Test MethodsMeasurement of Thermally Induced Discoloration for PTFE. The measured L*value and the % change in L* as calculated relative to Example 12—PTFEare shown in Table 4.

Examples 14, 15 and 16 PTFE—Drying with Ozone Enriched Air in thePresence of Alkali Metal Salt, KCl

The procedure of Example 13—PTFE is repeated except that the amount ofalkali metal salt, KCl, is increased to 0.515 gm, 0.687 gm and 0.762 gm,respectively. The measured L* value and the % change in L* as calculatedrelative to Example 12—PTFE are shown in Table 4.

The examples of Table 4 demonstrate the significant improvements in L*with the presence of alkali metal salt during the exposure of the wetfluoropolymer resin to an oxygen source during drying.

TABLE 4 Oxygen Source and Effect of Alkali Metal Salt Example O3-Aircc/min KCl, gm L* % Chg in L* Example 12 - PTFE 300 0 47.9 Example 13 -PTFE 300 0.145 66.7 47.7 Example 14 - PTFE 300 0.515 67.7 50.3 Example15 - PTFE 300 0.687 78.6 77.9 Example 16 - PTFE 300 0.762 80.5 82.7

Example 17 PTFE—Drying with Ozone Enriched Air

An aqueous PTFE dispersion is made as described above (PTFE—Preparationof Hydrocarbon Stabilized PTFE Dispersion). The resulting dispersion isdiluted to 15 wt % solids with deionized water and is isolated asdescribed above (Isolation of PTFE Dispersion) with the exception thatthe 600 grams of dispersion is 15 wt % solids rather than the 5 wt %solids described above. Approximately 55 gm of the wet polymer is thendried at 170° C. for 1 hour using the PTFE drier described above(Apparatus for Drying of PTFE Polymer) with the addition of ozoneenriched air. Ozone is produced by passing 100 cc/min of air into aClearWater Tech, LLC Model CD-10 ozone generator which is operated atthe full power setting. Dried polymer is characterized for thermallyinduced discoloration as described in the Test Methods Measurement ofThermally Induced Discoloration for PTFE. The measured L* value is 55.3and is shown in Table 5.

Example 18 PTFE—Drying with Ozone Enriched Air in the Presence of AlkaliMetal Salt, NaOH

An aqueous PTFE dispersion is made as described above (PTFE—Preparationof Hydrocarbon Stabilized PTFE Dispersion). A quantity of dispersion isdiluted to 15 wt % solids with deionized water. 0.135 gm of the alkalimetal salt, NaOH, is added to 600 gm of the dilute dispersion andthoroughly mixed. The resulting mixture is isolated as described above(Isolation of PTFE Dispersion) with the exception that the 600 grams ofdispersion is 15 wt % solids rather than the 5 wt % solids describedabove. Approximately 55 gm of the wet polymer is then dried at 170° C.for 1 hour using the PTFE drier described above (Apparatus for Drying ofPTFE Polymer) with the addition of ozone enriched air. Ozone is producedby passing 100 cc/min of air into a Clearwater Technologies, Inc. ModelCD-10 ozone generator which is operated at the full power setting. Driedpolymer is characterized for thermally induced discoloration asdescribed in the Test Methods Measurement of Thermally InducedDiscoloration for PTFE. The measured L* value and the % change in L* ascalculated relative to Example 17—PTFE are shown in Table 5.

Example 19 PTFE—Drying with Ozone Enriched Air in the Presence of AlkaliMetal Salt, NaCl

The procedure of Example 18—PTFE is repeated except that 0.114 gm ofNaCl is substituted for the NaOH. The measured L* value and the % changein L* as calculated relative to Example 17—PTFE are shown in Table 5.

Example 20 PTFE—Drying with Ozone Enriched Air in the Presence of AlkaliMetal Salt, NaCl

The procedure of Example 18—PTFE is repeated except that 0.195 gm ofNaCl is substituted for the NaOH. The measured L* value and the % changein L* as calculated relative to Example 17—PTFE are shown in Table 5.

Example 21 PTFE—Drying with Ozone Enriched Air in the Presence of AlkaliMetal Salt, LiCl

The procedure of Example 18—PTFE is repeated except that 0.083 gm ofLiCl is substituted for the NaOH. The measured L* value and the % changein L* as calculated relative to Example 17—PTFE are shown in Table 5.

Example 22 PTFE—Drying with Ozone Enriched Air in the Presence of AlkaliMetal Salt, LiCl

The procedure of Example 18—PTFE is repeated except that 0.468 gm ofLiCl is substituted for the NaOH. The measured L* value and the % changein L* as calculated relative to Example 17—PTFE are shown in Table 5.

Example 23 PTFE—Drying with Ozone Enriched Air in the Presence of AlkaliMetal Salt K₂CO₃

The procedure of Example 18—PTFE is repeated except that 0.150 gm ofK₂CO₃-1.5H₂O is substituted for the NaOH. The measured L* value and the% change in L* as calculated relative to Example 17—PTFE are shown inTable 5.

Example 24 PTFE—Drying with Ozone Enriched Air in the Presence of AlkaliMetal Salt, K₂SO₄

The procedure of Example 18—PTFE is repeated except that 0.171 gm ofK₂SO₄ is substituted for the NaOH. The measured L* value and the changein L* as calculated relative to Example 17—PTFE are shown in Table 5.

TABLE 5 Effect of Alkali Metal Salts Example Salt Salt, gm L* % Chg inL* Example 17 - PTFE 0 55.3 Example 18 - PTFE NaOH 0.135 83.2 87.2Example 19 - PTFE NaCl 0.114 80.6 79.1 Example 20 - PTFE NaCl 0.195 83.588.1 Example 21 - PTFE LiCl 0.083 64.2 27.8 Example 22 - PTFE LiCl 0.46880.2 80.9 Example 23 - PTFE K2CO3 - 1.5H2O 0.150 72.7 52.4 Example 24 -PTFE K2SO4 0.171 73.0 53.4

What is claimed is:
 1. Process for reducing thermally induceddiscoloration of fluoropolymer resin polymerized in the presence ofhydrocarbon surfactant wherein all of the monovalent substituents on thecarbon atoms of the hydrocarbon surfactant are hydrogen and wherein saidfluoropolymer resin has an initial thermally induced discoloration value(L_(i)) about 20 L units below the L value of equivalent fluoropolymerresin of commercial quality manufactured using ammoniumperfluorooctanoate fluorosurfactant, said process comprising:polymerizing fluoromonomer in an aqueous dispersion medium in thepresence of said hydrocarbon surfactant to form aqueous fluoropolymerdispersion having a raw dispersion particle size of 10 nm to 400 nm;isolating said fluoropolymer from said aqueous medium by separating wetfluoropolymer resin from the aqueous medium; drying said wetfluoropolymer resin to produce said fluoropolymer resin; and exposingsaid wet fluoropolymer resin to an oxygen source during drying, whereinsaid oxygen source is ozone containing gas.
 2. The process of claim 1wherein said process reduces thermally induced discoloration by at leastabout 10% as measured by % change in L* on the CIELAB color scale. 3.The process of claim 1 wherein said wet fluoropolymer resin is separatedfrom said dispersion by coagulating fluoropolymer from the aqueousfluoropolymer dispersion and filtering to remove the aqueous medium. 4.The process of claim 1 wherein said drying is carried out using dryinggas heated to a temperature range of about 100° C. to about 300° C. 5.The process of claim 1 wherein said fluoropolymer resin is PTFE resinand said drying is carried out using drying gas heated to a temperaturerange of about 100° C. to about 200° C.
 6. The process of claim 1wherein said fluoropolymer resin is melt-processible fluoropolymer resinand said drying is carried out said drying is carried out using dryinggas heated to a temperature range of about 160° C. to a temperatureabout 10° C. below the melting point of said fluoropolymer resin.
 7. Theprocess of claim 1 further comprising pretreating the aqueousfluoropolymer dispersion.
 8. The process of claim 7 wherein saidpretreating the aqueous fluoropolymer dispersion comprises exposing theaqueous fluoropolymer dispersion to oxidizing agent.
 9. The process ofclaim 7 wherein the reduction of thermally induced discolorationmeasured by % change in L* on the CIELAB color scale provided by saidpretreating in combination with exposing the wet fluoropolymer resin toan oxygen source during drying is at least about 10% greater than the %change in L* on the CIELAB color scale provided by only exposing saidwet fluoropolymer resin to an oxygen source under the same conditionsduring drying.
 10. The process of claim 1 wherein said exposing of saidwet fluoropolymer resin to an oxygen source during drying is carried outin the presence of alkali metal salt.
 11. The process of claim 10wherein said presence of alkali metal salt is provided by adding alkalimetal salt to the aqueous dispersion medium prior to separating said wetfluoropolymer resin from said aqueous dispersion medium.